mpls: Return error for RTA_GATEWAY attribute

[ Upstream commit be48220edd48ca0d569782992840488a52373a24 ]

MPLS does not support nexthops with an MPLS address family.
Specifically, it does not handle RTA_GATEWAY attribute. Make it
clear by returning an error.

Fixes: 03c05665 ("mpls: Netlink commands to add, remove, and dump routes")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>