ixv: Dynamically size the receive packet size

If MTU of the if_ixv VF driver is configured between 1501 and 2030, and
if a peer box have bigger MTU and sends a packet bigger than MTU into
this guest, kernel could crash.

Dynamically calculate the receive packet size on rx_mbuf_sz as ix(4)
does.

PR: 268092
Reported by: Kumara Babu <nkumarababu@gmail.com>
MFC after: 3 days
Sponsored by: BBOX.io