[XFRM]: CONFIG_XFRM_MIGRATE option

Add CONFIG_XFRM_MIGRATE option which makes it possible for for user
application to send or receive MIGRATE message to/from netlink socket.

Signed-off-by: Shinta Sugimoto <shinta.sugimoto@ericsson.com>
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>