ipv4: mpls: fix mpls_xmit for iptunnel

When using mpls over gre/gre6 setup, rt->rt_gw4 address is not set, the
same for rt->rt_gw_family.  Therefore, when rt->rt_gw_family is checked
in mpls_xmit(), neigh_xmit() call is skipped. As a result, such setup
doesn't work anymore.

This issue was found with LTP mpls03 tests.

Fixes: 1550c171

 ("ipv4: Prepare rtable for IPv6 gateway")
Signed-off-by: Alexey Kodanev <alexey.kodanev@oracle.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>