[XFS] Catch errors returned from xfs_bmap_last_offset().

xfs_bmap_last_offset() can fail and return an error.
xfs_iomap_write_allocate() fails to detect and propagate the error.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30802a

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