net80211: add IEEE80211_IS_LOCKED()

When trying to sort out a teardown locking problem (downcall in the
driver can sleep) I found that the fan-out tree was getting too big
to fix net80211 locking per-se for this while working on entirely
different problems.
Add IEEE80211_IS_LOCKED() so the driver can check and un-/re-lock as
necessary (as we do in other cases already) to avoid panics on
debug kernels left and right.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48475