btrfs: check correct variable after allocation in btrfs_backref_iter_alloc

The `if (!ret)` check will always be false and it may result in
ret->path being dereferenced while it is a NULL pointer.

Fixes: a37f232 ("btrfs: backref: introduce the skeleton of btrfs_backref_iter")
CC: [email protected] # 5.8+
Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Boleyn Su <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: BoleynSu

fs/btrfs/backref.c

@@ -2303,7 +2303,7 @@ struct btrfs_backref_iter *btrfs_backref_iter_alloc(
 		return NULL;
 
 	ret->path = btrfs_alloc_path();
-	if (!ret) {
+	if (!ret->path) {
 		kfree(ret);
 		return NULL;
 	}