selinux/nlmsg: add XFRM_MSG_MAPPING

This command is missing.

Fixes: 3a2dfbe8 ("xfrm: Notify changes in UDP encapsulation via netlink")
CC: Martin Willi <martin@strongswan.org>
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>