bcachefs: unlock parent dir if entry is not found in subvolume deletion

Parent dir is locked by user_path_locked_at() before validating the
required dentry. It should be unlocked if we can not perform the
deletion.

This fixes the problem:

$ bcachefs subvolume delete not-exist-entry
BCH_IOCTL_SUBVOLUME_DESTROY ioctl error: No such file or directory
$ bcachefs subvolume delete not-exist-entry

the second will stuck because the parent dir is locked in the previous
deletion.
Signed-off-by: Guoyu Ou <benogy@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-ioctl.c

@@ -455,6 +455,7 @@ static long bch2_ioctl_subvolume_destroy(struct bch_fs *c, struct file *filp,
 	if (IS_ERR(victim))
 		return PTR_ERR(victim);
 
+	dir = d_inode(path.dentry);
 	if (victim->d_sb->s_fs_info != c) {
 		ret = -EXDEV;
 		goto err;
@@ -463,14 +464,13 @@ static long bch2_ioctl_subvolume_destroy(struct bch_fs *c, struct file *filp,
 		ret = -ENOENT;
 		goto err;
 	}
-	dir = d_inode(path.dentry);
 	ret = __bch2_unlink(dir, victim, true);
 	if (!ret) {
 		fsnotify_rmdir(dir, victim);
 		d_delete(victim);
 	}
-	inode_unlock(dir);
 err:
+	inode_unlock(dir);
 	dput(victim);
 	path_put(&path);
 	return ret;