NFSv4: Move the DNS resolver into the NFSv4 module

The other protocols don't use it, so make it local to NFSv4, and
remove the EXPORT.
Also ensure that we only compile in cache_lib.o if we're using
the legacy DNS resolver.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Bryan Schumaker <bjschuma@netapp.com>