[XFS] Fix use after free in xfs_log_done().

The ticket allocation code got reworked in 2.6.26 and we now free tickets
whereas before we used to cache them so the use-after-free went
undetected.

SGI-PV: 985525

SGI-Modid: xfs-linux-melb:xfs-kern:31877a

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: David Chinner <david@fromorbit.com>