mds: prevent new wrlocks on LocalLock if there exists any xlock waiter

mds: prevent new wrlocks on LocalLock if there exists any xlock waiter

Otherwise, an xlock waiter can become starved as a LocalLock supports multiple
writers.

Strictly speaking, a new lock state would be appropriate for this but we cheat
frequently with the LocalLock -- there is only one state. All transition checks
are already manually performed by the Locker.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>