Fix a panic in softclock() if the interface is destroyed with a bpf consumer attached.
Fix a panic in softclock() if the interface is destroyed with a bpf consumer
attached.

This is caused by bpf_detachd clearing IFF_PROMISC on the interface which does
a SIOCSIFFLAGS ioctl. The problem here is that while the interface has been
stopped, IFF_UP has not been cleared so IFF_UP != IFF_DRV_RUNNING, this causes
the ioctl function to init() the interface which resets the callouts.

The destroy then completes and frees the softc but softclock will panic on a
dead callout pointer.

Ensure ifp->if_flags matches reality by clearing IFF_UP when we destroy.

Silence from:	rwatson
Approved by:	mlaier (mentor)
MFC after:	3 days

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