btrfs: return error pointer from alloc_test_extent_buffer

Callers of alloc_test_extent_buffer have not correctly interpreted the
return value as error pointer, as alloc_test_extent_buffer should behave
as alloc_extent_buffer. The self-tests were unaffected but
btrfs_find_create_tree_block could call both functions and that would
cause problems up in the call chain.

Fixes: faa2dbf004e8 ("Btrfs: add sanity tests for new qgroup accounting code")
CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index eb8bd02..2f4802f 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5074,12 +5074,14 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
 		return eb;
 	eb = alloc_dummy_extent_buffer(fs_info, start);
 	if (!eb)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 	eb->fs_info = fs_info;
 again:
 	ret = radix_tree_preload(GFP_NOFS);
-	if (ret)
+	if (ret) {
+		exists = ERR_PTR(ret);
 		goto free_eb;
+	}
 	spin_lock(&fs_info->buffer_lock);
 	ret = radix_tree_insert(&fs_info->buffer_radix,
 				start >> PAGE_SHIFT, eb);