ifnet: Detach BPF descriptors on interface vmove event

When an interface is moving to/from a vnet jail, it may still have BPF
descriptors attached. The userland (e.g. tcpdump) does not get noticed
that the interface is departing and still opens BPF descriptors thus
may result in leaking sensitive traffic (e.g. an interface is moved
back to parent jail but a user is still sniffing traffic over it in
the child jail).

Detach BPF descriptors so that the userland will be signaled.

Reviewed by: ae
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D45727