vfs: retire vnlru_under_unlocked

It only looks at the centralized value which in corner cases can end up
being negative.

Approved by: re (gjb)
(cherry picked from commit ba5dc166e0f390c703042910b637a83c705c8af3)
(cherry picked from commit b5210f3fb26031e24f2f4c465aa3eab64002353a)