ifnet/DrvAPI: Move if_t typedef to a better place

Summary:
<net/if_var.h> should really be used by the netstack only, not by
drivers.  Eventually all the accessors will be moved to <net/if.h> as
well, but for now just move the typedef while the KPI gets sorted and
drivers get converted.

Sponsored by:	Juniper Networks, Inc.
Reviewed By:	melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D37784

sys/net/if.h

@@ -657,6 +657,12 @@ MALLOC_DECLARE(M_IFADDR);
 MALLOC_DECLARE(M_IFMADDR);
 #endif
 
+/*
+ * Opaque interface structure.
+ */
+
+typedef struct ifnet * if_t;
+
 extern struct sx ifnet_detach_sxlock;
 
 struct nvlist;

sys/net/if_var.h

@@ -124,8 +124,6 @@ typedef enum {
 	IFCOUNTERS /* Array size. */
 } ift_counter;
 
-typedef struct ifnet * if_t;
-
 typedef	void (*if_start_fn_t)(if_t);
 typedef	int (*if_ioctl_fn_t)(if_t, u_long, caddr_t);
 typedef	void (*if_init_fn_t)(void *);