selinux: remove unneeded ipv6 hook wrappers

Netfilter places the protocol number the hook function is getting called
from in state->pf, so we can use that instead of an extra wrapper.

While at it, remove one-line wrappers too and make
selinux_ip_{out,forward,postroute} useable as hook function.

Signed-off-by: Florian Westphal <fw@strlen.de>
Message-Id: <20211011202229.28289-1-fw@strlen.de>
Signed-off-by: Paul Moore <paul@paul-moore.com>