Prevent USB network devices to generate a page fault trap when detached while UP and holding an open bpf handler by checking bpfilter_lookup() for returning NULL in bpfpoll().  Added an XXX comment which reminds us to recheck why this race condition happens in conjunction with the USB stack.
Prevent USB network devices to generate a page fault trap when detached
while UP and holding an open bpf handler by checking bpfilter_lookup()
for returning NULL in bpfpoll().  Added an XXX comment which reminds us
to recheck why this race condition happens in conjunction with the USB
stack.

Commented by miod@ and thib@ (would prefer to directly fix race condition,
if this is possible at all).

lot of help and OK claudio@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars