Btrfs: do not delete a subvolume which is in a R/O subvolume

Step to reproduce:
 # mkfs.btrfs <disk>
 # mount <disk> <mnt>
 # btrfs sub create <mnt>/subv0
 # btrfs sub snap <mnt> <mnt>/subv0/snap0
 # change <mnt>/subv0 from R/W to R/O
 # btrfs sub del <mnt>/subv0/snap0

We deleted the snapshot successfully. I think we should not be able to delete
the snapshot since the parent subvolume is R/O.
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>

fs/btrfs/ioctl.c

@@ -2095,12 +2095,12 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 		err = inode_permission(inode, MAY_WRITE | MAY_EXEC);
 		if (err)
 			goto out_dput;
+	}
 
-		/* check if subvolume may be deleted by a non-root user */
+	/* check if subvolume may be deleted by a user */
 	err = btrfs_may_delete(dir, dentry, 1);
 	if (err)
 		goto out_dput;
-	}
 
 	if (btrfs_ino(inode) != BTRFS_FIRST_FREE_OBJECTID) {
 		err = -EINVAL;