btrfs: fix extent buffer leak on failure to copy root

At btrfs_copy_root(), if the call to btrfs_inc_ref() fails we end up
returning without unlocking and releasing our reference on the extent
buffer named "cow" we previously allocated with btrfs_alloc_tree_block().

So fix that by unlocking the extent buffer and dropping our reference on
it before returning.

Fixes: be20aa9 ("Btrfs: Add mount option to turn off data cow")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -222,6 +222,8 @@ int btrfs_copy_root(struct btrfs_trans_handle *trans,
 	else
 		ret = btrfs_inc_ref(trans, root, cow, 0);
 	if (ret) {
+		btrfs_tree_unlock(cow);
+		free_extent_buffer(cow);
 		btrfs_abort_transaction(trans, ret);
 		return ret;
 	}