configfs: Fix inconsistent use of file_inode() vs file->f_path.dentry->d_inode

Fix inconsistent use of file_inode() vs file->f_path.dentry->d_inode.
Reported-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid Howells <dhowells@redhat.com>
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/configfs/dir.c

@@ -1598,7 +1598,7 @@ static loff_t configfs_dir_lseek(struct file *file, loff_t offset, int whence)
 			if (offset >= 0)
 				break;
 		default:
-			mutex_unlock(&file_inode(file)->i_mutex);
+			mutex_unlock(&dentry->d_inode->i_mutex);
 			return -EINVAL;
 	}
 	if (offset != file->f_pos) {