Merge branch 'ipv4-Enable-support-for-IPv6-gateway-with-IPv4-routes'

David Ahern says:

====================
ipv4: Enable support for IPv6 gateway with IPv4 routes

Last set of three with the end goal of enabling IPv6 gateways with IPv4
routes.

This set adds fib6_nh_init and release to the IPv6 stubs, and adds neighbor
helpers that IPv4 code invokes to resolve an IPv6 address. When using
an IPv6 neighbor entry the hh_cache is bypassed as it contains the wrong
ethernet header for an IPv4 packet.

The nh_common nhc_has_gw was a temporary field used to convert existing
code from fib{6}_nh to fib_nh_common. That field is now converted to
nhc_gw_family to differentiate the address family of the gateway entry
as opposed to the address family of the container of fib_nh_common.

Existing code for rtable and fib_config is refactored to prepare
for a v6 address and then support is added. From there various
miscellaneous functions are updated to handle a v6 gateway - from
validating the v6 address to lookups in bpf code to verifying the
nexthop state.

Offload drivers - mlxsw and rocker - are modified to detect the v6
gateway and reject the route as 'unsupported'. e.g.,

    $ ip ro add 172.16.101.0/24 via inet6 fe80::202:ff:fe00:b dev swp1s0
    Error: mlxsw_spectrum: IPv6 gateway with IPv4 route is not supported.

This can be removed in time once support is added to each.

With the infrastructure changes in place, patch 17 enables it by adding
support for RTA_VIA to IPv4. RTA_VIA can be used for IPv4 addresses as
well. Only one of RTA_VIA and RTA_GATEWAY can be passed in a request.

Patch 18 adds a few test cases to fib_tests.sh.

v2
- comments from Ido - fixed typos as noted and updated messages
- add commit message to patch 1
- In patch 9, ipv4: Add fib_check_nh_v6_gw, moved the call to
  fib6_nh_release under the 'if (!err)' check as the intention is
  that release should not be called if init fails.
====================

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