[XFS] Fix block reservation changes for non-SMP systems.

SGI-PV: 956323
SGI-Modid: xfs-linux-melb:xfs-kern:27940a

Signed-off-by: Kevin Jamieson <kjamieson@bycast.com>
Signed-off-by: David Chatterton <chatz@sgi.com>
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>