common/ceph_mutex: note whether mutex debug methods are usable

common/ceph_mutex: note whether mutex debug methods are usable

So we can do checks like:

    ceph_assert(!ceph::mutex_debugging || !ceph_mutex_is_locked_by_me(lock));

Without this boolean, this check:

    ceph_assert(!ceph_mutex_is_locked_by_me(lock));

will fail for all crimson/release builds because the method always returns true.

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