[XFRM] STATE: Common receive function for route optimization extension headers.

XFRM_STATE_WILDRECV flag is introduced; the last resort state is set
it and receives packet which is not route optimized but uses such
extension headers i.e. Mobile IPv6 signaling (binding update and
acknowledgement).  A node enabled Mobile IPv6 adds the state.

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>