vfs cache: sanitize debug counters

They are very rarely triggered, so no need for per-cpu distribution.

At the same time the non-cpu ones still should use atomics to not lose
any updates.

(cherry picked from commit 2749c222da8a6325d39c0571f72b1dbed2f7d583)
(cherry picked from commit 75b03f73b4a0f94090aa9fd050e82089c83fe2bb)

Approved by:re (gjb)