[XFS] Check for errors when changing buffer pointers.

xfs_buf_associate_memory() can fail, but the return is never checked.
Propagate the error through XFS_BUF_SET_PTR() so that failures are
detected.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30831a

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