Now that all users of the sema_t are gone from XFS we can finally kill it.

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31823a
Signed-off-by: NDavid Chinner <david@fromorbit.com>
Signed-off-by: NLachlan McIlroy <lachlan@sgi.com>