selinux/nlmsg: add XFRM_MSG_MIGRATE

[ Upstream commit 8d465bb777179c4bea731b828ec484088cc9fbc1 ]

This command is missing.

Fixes: 5c79de6e ("[XFRM]: User interface for handling XFRM_MSG_MIGRATE")
Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>