[XFRM] STATE: Introduce route optimization mode.

Route optimization is used with routing header and destination options
header for Mobile IPv6.

At outbound it makes header space like IPsec transport. At inbound it
does nothing because exhdrs.c functions have responsibility to update
skbuff information for these headers.

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>