Convert page cache read to VOP.

There are several negative side-effects of not calling into VOP layer
at all for page cache reads. The biggest is the missed activation of
EVFILT_READ knotes.

Also, it allows filesystem to make more fine grained decision to
refuse read from page cache.

Keep VIRF_PGREAD flag around, it is still useful for nullfs, and for
asserts.

Reviewed by: markj
Tested by: pho
Discussed with: mjg
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D26346