SUNRPC: ip map cache per network namespace cleanup

This patch converts ip_map_cache per network namespace implemenetation to the
same view, as other caches done in the series.
Besides generalization, code becomes shorter with this patch.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: J. Bruce Fields <bfields@redhat.com>