bcachefs: Drop redundant btree_path_downgrade()s

If a path doesn't have any active references, we shouldn't downgrade it;
it'll either be reused, possibly with intent refs again, or dropped at
bch2_trans_begin() time.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_locking.c

@@ -747,6 +747,7 @@ void bch2_trans_downgrade(struct btree_trans *trans)
 		return;
 
 	trans_for_each_path(trans, path, i)
+		if (path->ref)
 			bch2_btree_path_downgrade(trans, path);
 }