[XFRM] MIP6: Fix address keys for routing search.

Each MIPv6 XFRM state (DSTOPT/RH2) holds either destination or source
address to be mangled in the IPv6 header (that is "CoA").
On Inter-MN communication after both nodes binds each other,
they use route optimized traffic two MIPv6 states applied, and
both source and destination address in the IPv6 header
are replaced by the states respectively.
The packet format is correct, however, next-hop routing search
are not.
This patch fixes it by remembering address pairs for later states.

Based on patch from Masahide NAKAMURA <nakam@linux-ipv6.org>.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>