[XFS] Need to unlock the AIL before calling xfs_force_shutdown() because

when it goes to force out the log, and get the tail lsn, it will want to
get the AIL lock.

SGI-PV: 940076
SGI-Modid: xfs-linux:xfs-kern:23260a

Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>