xfs: Remove XFS_MOUNT_RETERR

XFS_MOUNT_RETERR is going to be set at xfs_parseargs() if
mp->m_dalign is enabled, so any time we enter "if (mp->m_dalign)"
branch in xfs_update_alignment(), XFS_MOUNT_RETERR is set and so
we always be emitting a warning and returning an error.

Hence, we can remove it and get rid of a couple of redundant
check up against it at xfs_upate_alignment().

Thanks Dave Chinner for the suggestions of simplify the code
in xfs_parseargs().

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>