[XFS] Clean up quotamount error handling.

xfs_qm_mount_quotas() returns an error status that is ignored. If we fail
to mount quotas, we continue with quota's turned off, which is all handled
inside xfs_qm_mount_quotas(). Mark it as void to indicate that errors need
not be returned to the callers.

SGI-PV: 980084
SGI-Modid: xfs-linux-melb:xfs-kern:30788a

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