[XFS] Don't error out on good I/Os.

xfsbdstrat() made all I/Os error out, good or bad. Fix it.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30836a

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