vfs cache: garbage collect the fullpathfail2 counter

The conditions it checks cannot legally be true (modulo races against
forced unmount), so assert on it instead.

Approved by: re (gjb)
(cherry picked from commit 07f52c4b5a4f7a658bf7ed29eadd5c4d52a4f3b2)
(cherry picked from commit 8e4a043bad750d85e0028ae6ceb9c9e37842bf4a)