vfs: add max_vnlru_free to the vfs.vnode.vnlru tree

While here rename the var internally.

(cherry picked from commit 281a9715b582861fe4097c2f27eb27b208d752b1)
(cherry picked from commit 14babcc0815e74079d608f4fe1bc1892107e08a5)

Approved by: re (gjb)