Have acl_execute_masked() take an acl_t rather than a path as its argument.

We only call acl_execute_masked() in one place; and in that place, the
ACL of the path in question is already available. So, there's no
reason for us to re-retrieve it. Instead, the function has been
updated to take an acl_t (and not a path), simplifing the logic a bit.