[IPV6] MIP6: Add socket option and ancillary data interface of routing header type 2.

Add socket option and ancillary data interface of routing header type
2.  Mobile IPv6 application will use this to send binding
acknowledgement with the header without relation of confirmed route
optimization (binding).

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>