Btrfs: lock superblock before remounting for rw subvol

Since commit 0723a047 ("btrfs: allow mounting btrfs subvolumes with
different ro/rw options"), when mounting a subvolume read/write when
another subvolume has previously been mounted read-only, we first do a
remount. However, this should be done with the superblock locked, as per
sync_filesystem():

	/*
	 * We need to be protected against the filesystem going from
	 * r/o to r/w or vice versa.
	 */
	WARN_ON(!rwsem_is_locked(&sb->s_umount));

This WARN_ON can easily be hit with:

mkfs.btrfs -f /dev/vdb
mount /dev/vdb /mnt
btrfs subvol create /mnt/vol1
btrfs subvol create /mnt/vol2
umount /mnt
mount -oro,subvol=/vol1 /dev/vdb /mnt
mount -orw,subvol=/vol2 /dev/vdb /mnt2

Fixes: 0723a047 ("btrfs: allow mounting btrfs subvolumes with different ro/rw options")
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NOmar Sandoval <osandov@osandov.com>
Signed-off-by: NChris Mason <clm@fb.com>

fs/btrfs/super.c

@@ -1209,7 +1209,9 @@ static struct dentry *mount_subvol(const char *subvol_name, int flags,
 				return ERR_CAST(mnt);
 			}
 
+			down_write(&mnt->mnt_sb->s_umount);
 			r = btrfs_remount(mnt->mnt_sb, &flags, NULL);
+			up_write(&mnt->mnt_sb->s_umount);
 			if (r < 0) {
 				/* FIXME: release vfsmount mnt ??*/
 				kfree(newargs);