MFC r356633:
Make sure the VNET is properly set when reaping mbufs in ipoib.
Else the following panic may happen:

panic()
icmp_error()
ipoib_cm_mb_reap()
linux_work_fn()
taskqueue_run_locked()
taskqueue_thread_loop()
fork_exit()
fork_trampoline()

Submitted by: Andreas Kempe <kempe@lysator.liu.se>
Sponsored by: Mellanox Technologies