Use buffer pager for NFS.

The pager, due to its construction, implements clustering for the
page-ins. In particular, buildworld load demonstrates reduction of
the READ RPCs from 39k down to 24k. No change in real or CPU time was
observed.

Discussed with, and measured by: bde
No objections from: rmacklem
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week