Factor the remount read only code into a helper to simplify the
subsequent change from the super block method .remount_fs to the
mount-api fs_context_operations method .reconfigure.
Signed-off-by: Ian Kent <raven@themaw.net> Reviewed-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>