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: a37f232b ("btrfs: backref: introduce the skeleton of btrfs_backref_iter")
CC: stable@vger.kernel.org # 5.8+
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boleyn Su <boleynsu@google.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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;
 	}