btrfs: fix silent failure when deleting root reference

At btrfs_del_root_ref(), if btrfs_search_slot() returns an error, we end
up returning from the function with a value of 0 (success). This happens
because the function returns the value stored in the variable 'err',
which is 0, while the error value we got from btrfs_search_slot() is
stored in the 'ret' variable.

So fix it by setting 'err' with the error value.

Fixes: 8289ed9f93bef2 ("btrfs: replace the BUG_ON in btrfs_del_root_ref with proper error handling")
CC: [e-mail] # 5.16+
Reviewed-by: Qu Wenruo <[e-mail]>
Signed-off-by: Filipe Manana <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/root-tree.c

···
 	key.offset = ref_id;
 again:
 	ret = btrfs_search_slot(trans, tree_root, &key, path, -1, 1);
-	if (ret < 0)
+	if (ret < 0) {
+		err = ret;
 		goto out;
-	if (ret == 0) {
+	} else if (ret == 0) {
 		leaf = path->nodes[0];
 		ref = btrfs_item_ptr(leaf, path->slots[0],
 				     struct btrfs_root_ref);