On uniprocessor machines, the incore superblock is used for all in memory
accounting of free blocks. in this situation, changes to the reserved
block count are accounted twice; once directly and once via
xfs_mod_incore_sb(). Seeing as the modification on SMP is done via
xfs_mod_incore_sb(), make this the only update mechanism that UP uses as
well.

SGI-PV: 980654
SGI-Modid: xfs-linux-melb:xfs-kern:30997a
Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>