ipv6: use net->rt_genid to check dst validity

IPv6 dst should take care of rt_genid too. When a xfrm policy is inserted or
deleted, all dst should be invalidated.
To force the validation, dst entries should be created with ->obsolete set to
DST_OBSOLETE_FORCE_CHK. This was already the case for all functions calling
ip6_dst_alloc(), except for ip6_rt_copy().

As a consequence, we can remove the specific code in inet6_connection_sock.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>