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)
(cherry picked from commit cfbc3927613a8455498db5ff3e67046a4dab15a1)

Approved by: re (gjb)