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>