[XFRM] STATE: Introduce care-of address.

Care-of address is carried by state as a transformation option like
IPsec encryption/authentication algorithm.

Based on MIPL2 kernel patch.

Signed-off-by: Noriaki TAKAMIYA <takamiya@po.ntts.co.jp>
Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>