[XFS] Ensure errors from xfs_bdstrat() are correctly checked.

xfsbdstrat() is declared to return an error. That is never checked because
the error is propagated by the xfs_buf_t that is passed through the
function.

Mark xfsbdstrat() as returning void and comment the prototype on the
methods needed for error checking.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30823a

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