[XFS] Reorganise xlog_t for better cacheline isolation of contention

To reduce contention on the log in large CPU count, separate out different
parts of the xlog_t structure onto different cachelines. Move each lock
onto a different cacheline along with all the members that are
accessed/modified while that lock is held.

Also, move the debugging code into debug code.

SGI-PV: 978729
SGI-Modid: xfs-linux-melb:xfs-kern:30772a

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