[IPV6] IPSEC: Support sending with Mobile IPv6 extension headers.

Mobile IPv6 defines home address option as an option of destination
options header. It is placed before fragment header then
ip6_find_1stfragopt() is fixed to know about it.

Home address option also carries final source address of the flow,
then outbound AH calculation should take care of it like routing
header case.  Based on MIPL2 kernel patch.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>