The assert was meant as an additional check to the assert in link.c:198.
The reasoning behind the assert in link.c:198 is that once you've
obtained a write lock on a vmnt, you can't get an additional read lock
on the same vmnt. However, that does not always hold for the assert in
path.c:281 where the situation could be that you've obtained a read lock
and managed to get another read lock (this is possible). In other words,
the assert in path.c:281 is not the right place to check for that
situation.