btrfs: hold a ref on the root in __btrfs_run_defrag_inode

We are looking up an arbitrary inode, we need to hold a ref on the root
while we're doing this.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/file.c

@@ -292,11 +292,16 @@ static int __btrfs_run_defrag_inode(struct btrfs_fs_info *fs_info,
 		ret = PTR_ERR(inode_root);
 		goto cleanup;
 	}
+	if (!btrfs_grab_fs_root(inode_root)) {
+		ret = -ENOENT;
+		goto cleanup;
+	}
 
 	key.objectid = defrag->ino;
 	key.type = BTRFS_INODE_ITEM_KEY;
 	key.offset = 0;
 	inode = btrfs_iget(fs_info->sb, &key, inode_root);
+	btrfs_put_fs_root(inode_root);
 	if (IS_ERR(inode)) {
 		ret = PTR_ERR(inode);
 		goto cleanup;