btrfs: call mnt_drop_write after interrupted subvol deletion

If btrfs_ioctl_snap_destroy blocks on the mutex and the process is
killed, mnt_write count is unbalanced and leads to unmountable
filesystem.

CC: stable@vger.kernel.org
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ioctl.c

@@ -2121,7 +2121,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 
 	err = mutex_lock_killable_nested(&dir->i_mutex, I_MUTEX_PARENT);
 	if (err == -EINTR)
-		goto out;
+		goto out_drop_write;
 	dentry = lookup_one_len(vol_args->name, parent, namelen);
 	if (IS_ERR(dentry)) {
 		err = PTR_ERR(dentry);
@@ -2284,6 +2284,7 @@ out_dput:
 	dput(dentry);
 out_unlock_dir:
 	mutex_unlock(&dir->i_mutex);
+out_drop_write:
 	mnt_drop_write_file(file);
 out:
 	kfree(vol_args);