Hyper-V: hn: Initialize the internal field of per packet info on tx path

The RSC support feature introduced a bit field "rm_internal" in
struct rndis_pktinfo with total size unchanged.

The guest does not use this field in the tx path. However we need to
initialize it to zero in case older hosts which are not aware of this
field.

Fixes:		a491581f ("Hyper-V: hn: Enable vSwitch RSC support")
MFC after:	2 weeks
Sponsored by:	Microsoft

sys/dev/hyperv/netvsc/if_hn.c

@@ -2932,6 +2932,7 @@ hn_rndis_pktinfo_append(struct rndis_packet_msg *pkt, size_t pktsize,
 
 	pi->rm_size = pi_size;
 	pi->rm_type = pi_type;
+	pi->rm_internal = 0;
 	pi->rm_pktinfooffset = RNDIS_PKTINFO_OFFSET;
 
 	return (pi->rm_data);