selinux: fix a problem with IPv6 traffic denials in selinux_ip_postroute()

A previous commit c0828e50

 ("selinux:
process labeled IPsec TCP SYN-ACK packets properly in
selinux_ip_postroute()") mistakenly left out a 'break' from a switch
statement which caused problems with IPv6 traffic.

Thanks to Florian Westphal for reporting and debugging the issue.

Reported-by: Florian Westphal <fwestpha@redhat.com>
Signed-off-by: Paul Moore <pmoore@redhat.com>