selinux: handle ICMPv6 consistently with ICMP

commit 79c8b348f215 ("selinux: support distinctions among all network
address families") mapped datagram ICMP sockets to the new icmp_socket
security class, but left ICMPv6 sockets unchanged.  This change fixes
that oversight to handle both kinds of sockets consistently.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>