enforce ->sync_fs is only called for rw superblock

Make sure a superblock really is writeable by checking MS_RDONLY
under s_umount.  sync_filesystems needed some re-arragement for
that, but all but one sync_filesystem caller had the correct locking
already so that we could add that check there.  cachefiles grew
s_umount locking.

I've also added a WARN_ON to sync_filesystem to assert this for
future callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/reiserfs/super.c b/fs/reiserfs/super.c
index 1b52daa..3da0401c 100644
--- a/fs/reiserfs/super.c
+++ b/fs/reiserfs/super.c
@@ -64,18 +64,15 @@
 
 static int reiserfs_sync_fs(struct super_block *s, int wait)
 {
-	if (!(s->s_flags & MS_RDONLY)) {
-		struct reiserfs_transaction_handle th;
-		reiserfs_write_lock(s);
-		if (!journal_begin(&th, s, 1))
-			if (!journal_end_sync(&th, s, 1))
-				reiserfs_flush_old_commits(s);
-		s->s_dirt = 0;	/* Even if it's not true.
-				 * We'll loop forever in sync_supers otherwise */
-		reiserfs_write_unlock(s);
-	} else {
-		s->s_dirt = 0;
-	}
+	struct reiserfs_transaction_handle th;
+
+	reiserfs_write_lock(s);
+	if (!journal_begin(&th, s, 1))
+		if (!journal_end_sync(&th, s, 1))
+			reiserfs_flush_old_commits(s);
+	s->s_dirt = 0;	/* Even if it's not true.
+			 * We'll loop forever in sync_supers otherwise */
+	reiserfs_write_unlock(s);
 	return 0;
 }