vfs cache: Add a predicate for testing cache entries

No functional change intended.

Reviewed by: olce, kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D50105

(cherry picked from commit 029ed5f574f61443fd12e0e70c7ab4dc296b81e5)