Extend PF_KEYv2 framework so that user application can take advantage
of MIGRATE feature via PF_KEYv2 interface. User application can either
send or receive an MIGRATE message to/from PF_KEY socket.

Detail information can be found in the internet-draft
<draft-sugimoto-mip6-pfkey-migrate>.
Signed-off-by: NShinta Sugimoto <shinta.sugimoto@ericsson.com>
Signed-off-by: NMasahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>