mpls: fix rtm policy in mpls_getroute

fix rtm policy name typo in mpls_getroute and also remove
export of rtm_ipv4_policy

Fixes: 397fc9e5

 ("mpls: route get support")
Reported-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>