Include cache zones into zone_foreach() where appropriate.

The r354367 is reverted since it is subsumed by this, more complete, approach.

Suggested by: markj
Reviewed by: alc. glebius, markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D22242