The advent of if_detach, allowing interface removal at runtime, makes it

possible for a panic to occur if BPF is in use on the interface at the
time of the call to if_detach.  This happens because BPF maintains pointers
to the struct ifnet describing the interface, which is freed by if_detach.

To correct this problem, a new call, bpfdetach, is introduced.  bpfdetach
locates BPF descriptor references to the interface, and NULLs them.  Other
BPF code is modified so that discovery of a NULL interface results in
ENXIO (already implemented for some calls).  Processes blocked on a BPF
call will also be woken up so that they can receive ENXIO.

Interface drivers that invoke bpfattach and if_detach must be modified to
also call bpfattach(ifp) before calling if_detach(ifp).  This is relevant
for buses that support hot removal, such as pccard and usb.  Patches to
all effected devices will not be committed, only to if_wi.c, due to
testing limitations.  To reproduce the crash, load up tcpdump on you
favorite pccard ethernet card, and then eject the card.  As some pccard
drivers do not invoke if_detach(ifp), this bug will not manifest itself
for those drivers.

Reviewed by:	wes

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