libfs: Add a lock class for the offset map's xa_lock

Tie the dynamically-allocated xarray locks into a single class so
contention on the directory offset xarrays can be observed.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Message-Id: <169020933088.160441.9405180953116076087.stgit@manet.1015granger.net>
Signed-off-by: Christian Brauner <brauner@kernel.org>