xfs: disallow ro->rw remount on norecovery mount

There's a bit of a loophole in norecovery mount handling right
now: an initial mount must be readonly, but nothing prevents
a mount -o remount,rw from producing a writable, unrecovered
xfs filesystem.

It might be possible to try to perform a log recovery when this
is requested, but I'm not sure it's worth the effort.  For now,
simply disallow this sort of transition.
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_super.c

@@ -1237,6 +1237,12 @@ xfs_fs_remount(
 
 	/* ro -> rw */
 	if ((mp->m_flags & XFS_MOUNT_RDONLY) && !(*flags & MS_RDONLY)) {
+		if (mp->m_flags & XFS_MOUNT_NORECOVERY) {
+			xfs_warn(mp,
+		"ro->rw transition prohibited on norecovery mount");
+			return -EINVAL;
+		}
+
 		mp->m_flags &= ~XFS_MOUNT_RDONLY;
 
 		/*