[XFS] fix logic error in xfs_alloc_ag_vextent_near()

Fix a logic error in xfs_alloc_ag_vextent_near(). This is a regression
introduced by the error handling changes.

SGI-PV: 890084
SGI-Modid: xfs-linux-melb:xfs-kern:30838a

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