Merge branch 'net-fail-route'

David Ahern says:

====================
net: Fail route add with unsupported nexthop attribute

RTA_VIA was added for MPLS as a way of specifying a gateway from a
different address family. IPv4 and IPv6 do not currently support RTA_VIA
so using it leads to routes that are not what the user intended. Catch
and fail - returning a proper error message.

MPLS on the other hand does not support RTA_GATEWAY since it does not
make sense to have a nexthop from the MPLS address family. Similarly,
catch and fail - returning a proper error message.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>