SELinux: Fix a RCU free problem with the netport cache

The netport cache doesn't free resources in a manner which is safe or orderly.
This patch fixes this by adding in a missing call to rcu_dereference() in
sel_netport_insert() as well as some general cleanup throughout the file.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>