[XFRM]: User interface for handling XFRM_MSG_MIGRATE

Add user interface for handling XFRM_MSG_MIGRATE. The message is issued
by user application. When kernel receives the message, procedure of
updating XFRM databases will take place.

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>