xfrm: Accept XFRM_STATE_AF_UNSPEC SAs on IPv4/IPv6 only hosts

Installing SAs using the XFRM_STATE_AF_UNSPEC fails on hosts with
support for one address family only. This patch accepts such SAs, even
if the processing of not supported packets will fail.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: David S. Miller <davem@davemloft.net>