[XFRM] STATE: Add a hook to obtain local/remote outbound address.

Outbound transformation replaces both source and destination address with
state's end-point addresses at the same time when IPsec tunnel mode.
It is also required to change them for Mobile IPv6 route optimization, but we
should care about the following differences:
 - changing result is not end-point but care-of address
 - either source or destination is replaced for each state
This hook is a common platform to change outbound address.
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>