Merge branch 'mpls-build-fix'

Roopa Prabhu says:

====================
af_mpls: fix undefined reference to ip6_route_output with CONFIG_IPV6=n

This patch series uses ipv6_stub_impl.ipv6_dst_lookup instead of
ip6_route_output. Follows the vxlan drivers usage of
ipv6_stub_impl.ipv6_dst_lookup.

There is no sk in the af_mpls context from where
ipv6_stub_impl.ipv6_dst_lookup is used. sk appears to be needed
to get the namespace 'net' and is optional otherwise. This patch series
changes ipv6_stub_impl.ipv6_dst_lookup to take net argument. sk remains
optional.

v1 - v2: use IS_BUILTIN

v2 - v3: Use new Kconfig option that depends on (IPV6 || IPV6=n) as
	 suggested by Dave. Also uses IS_ERR as suggested by Thomas.

v3 - v4: Include missed case of (MPLS_ROUTING=y && IPV6=m) reported by
         Dave.

v4 - v5: Use ipv6_stub_impl.ipv6_dst_lookup as suggested by Hannes

v5 - v6: protect against null ipv6_stub by statically declaring
	 a ipv6_dst_lookup NOP func
====================

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