hyperv/hn: Implement transparent mode network VF.

How network VF works with hn(4) on Hyper-V in transparent mode:

- Each network VF has a cooresponding hn(4).
- The network VF and the it's cooresponding hn(4) have the same hardware
  address.
- Once the network VF is attached, the cooresponding hn(4) waits several
  seconds to make sure that the network VF attach routing completes, then:
  o  Set the intersection of the network VF's if_capabilities and the
     cooresponding hn(4)'s if_capabilities to the cooresponding hn(4)'s
     if_capabilities.  And adjust the cooresponding hn(4) if_capable and
     if_hwassist accordingly. (*)
  o  Make sure that the cooresponding hn(4)'s TSO parameters meet the
     constraints posed by both the network VF and the cooresponding hn(4).
     (*)
  o  The network VF's if_input is overridden.  The overriding if_input
     changes the input packet's rcvif to the cooreponding hn(4).  The
     network layers are tricked into thinking that all packets are
     neceived by the cooresponding hn(4).
  o  If the cooresponding hn(4) was brought up, bring up the network VF.
     The transmission dispatched to the cooresponding hn(4) are
     redispatched to the network VF.
  o  Bringing down the cooresponding hn(4) also brings down the network
     VF.
  o  All IOCTLs issued to the cooresponding hn(4) are pass-through'ed to
     the network VF; the cooresponding hn(4) changes its internal state
     if necessary.
  o  The media status of the cooresponding hn(4) solely relies on the
     network VF.
  o  If there are multicast filters on the cooresponding hn(4), allmulti
     will be enabled on the network VF. (**)
- Once the network VF is detached.  Undo all damages did to the
  cooresponding hn(4) in the above item.

NOTE:
No operation should be issued directly to the network VF, if the
network VF transparent mode is enabled.  The network VF transparent mode
can be enabled by setting tunable hw.hn.vf_transparent to 1.  The network
VF transparent mode is _not_ enabled by default, as of this commit.

The benefit of the network VF transparent mode is that the network VF
attachment and detachment are transparent to all network layers; e.g. live
migration detaches and reattaches the network VF.

The major drawbacks of the network VF transparent mode:
- The netmap(4) support is lost, even if the VF supports it.
- ALTQ does not work, since if_start method cannot be properly supported.

(*)
These decisions were made so that things will not be messed up too much
during the transition period.

(**)
This does _not_ need to go through the fancy multicast filter management
stuffs like what vlan(4) has, at least currently:
- As of this write, multicast does not work in Azure.
- As of this write, multicast packets go through the cooresponding hn(4).

MFC after:	3 days
Sponsored by:	Microsoft
Differential Revision:	https://reviews.freebsd.org/D11803

sys/dev/hyperv/netvsc/if_hnreg.h

@@ -133,7 +133,7 @@ struct hn_nvs_ndis_init {
 } __packed;
 CTASSERT(sizeof(struct hn_nvs_ndis_init) >= HN_NVS_REQSIZE_MIN);
 
-#define HN_NVS_DATAPATH_SYNTHETIC	0
+#define HN_NVS_DATAPATH_SYNTH		0
 #define HN_NVS_DATAPATH_VF		1
 
 /* No response */

sys/dev/hyperv/netvsc/if_hnvar.h

@@ -174,7 +174,6 @@ struct hn_tx_ring {
  */
 struct hn_softc {
 	struct ifnet    *hn_ifp;
-	struct ifnet	*hn_vf_ifp;	/* SR-IOV VF */
 	struct ifmedia	hn_media;
 	device_t        hn_dev;
 	int             hn_if_flags;
@@ -185,6 +184,10 @@ struct hn_softc {
 	int		hn_rx_ring_inuse;
 	struct hn_rx_ring *hn_rx_ring;
 
+	struct rmlock	hn_vf_lock;
+	struct ifnet	*hn_vf_ifp;	/* SR-IOV VF */
+	uint32_t	hn_xvf_flags;	/* transparent VF flags */
+
 	int		hn_tx_ring_cnt;
 	int		hn_tx_ring_inuse;
 	struct hn_tx_ring *hn_tx_ring;
@@ -241,6 +244,24 @@ struct hn_softc {
 	eventhandler_tag	hn_ifnet_evthand;
 	eventhandler_tag	hn_ifnet_atthand;
 	eventhandler_tag	hn_ifnet_dethand;
+	eventhandler_tag	hn_ifnet_lnkhand;
+
+	/*
+	 * Transparent VF delayed initialization.
+	 */
+	int			hn_vf_rdytick;	/* ticks, 0 == ready */
+	struct taskqueue	*hn_vf_taskq;
+	struct timeout_task	hn_vf_init;
+
+	/*
+	 * Saved information for VF under transparent mode.
+	 */
+	void			(*hn_vf_input)
+				(struct ifnet *, struct mbuf *);
+	int			hn_saved_caps;
+	u_int			hn_saved_tsomax;
+	u_int			hn_saved_tsosegcnt;
+	u_int			hn_saved_tsosegsz;
 };
 
 #define HN_FLAG_RXBUF_CONNECTED		0x0001
@@ -255,6 +276,9 @@ struct hn_softc {
 
 #define HN_FLAG_ERRORS			(HN_FLAG_RXBUF_REF | HN_FLAG_CHIM_REF)
 
+#define HN_XVFFLAG_ENABLED		0x0001
+#define HN_XVFFLAG_ACCBPF		0x0002
+
 #define HN_NO_SLEEPING(sc)			\
 do {						\
 	(sc)->hn_flags |= HN_FLAG_NO_SLEEPING;	\