structure is zeroed, by setting the VNET after checking the mbuf count
for zero. It appears there are some cases with early interrupts on some
network devices which still trigger page-faults on accessing a NULL "ifp"
pointer before the TCP LRO control structure has been initialized.
This basically preserves the old behaviour, prior to
9ca874cf .

No functional change.

Reported by:	rscheff@
Differential Revision:	https://reviews.freebsd.org/D29564
MFC after:	2 weeks
Sponsored by:	Mellanox Technologies // NVIDIA Networking