LinuxKPI: 802.11: use an sx lock to protect the list of vifs

Use an sx lock to protect the list of vifs. We could use the
linux mutex compat for this but our current implementation may
re-acquire the lock recursively so allow this. The change is
mainly motivated by the fact that some callers may sleep in the
interator function called. Recursiveness is needed because we
see find_sta_by_ifaddr() being called from an iterator function
from iterate_interfaces().

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