btrfs: don't BUG_ON on ENOMEM from btrfs_lookup_extent_info() in walk_down_proc()

We handle errors here properly, ENOMEM isn't fatal, return the error.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -5397,7 +5397,6 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 					       &wc->refs[level],
 					       &wc->flags[level],
 					       NULL);
-		BUG_ON(ret == -ENOMEM);
 		if (ret)
 			return ret;
 		BUG_ON(wc->refs[level] == 0);