vfs: prefix regular vnlru with a special case for free vnodes

Works around severe performance problems in certain corner cases, see
the commentary added.

Modifying vnlru logic has proven rather error prone in the past and a
release is near, thus take the easy way out and fix it without having to
dig into the current machinery.

(cherry picked from commit 90a008e94bb205e5b8f3c41d57e155b59a6be95d)