[XFS] Remove useless memory barrier

wake_up's implementation does an implicit memory barrier so the explicit
memory barrier is not needed in vfs_sync_worker.

Patch provided by Ralf Baechle.

SGI-PV: 960867
SGI-Modid: xfs-linux-melb:xfs-kern:28032a

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>