netfilter: nf_nat: in nf_nat_initialized(), use const struct nf_conn *

nf_nat_initialized() doesn't modify passed struct nf_conn,
so declare as const.

This is helpful for code readability and makes it possible
to call nf_nat_initialized() with a const struct nf_conn *.

Signed-off-by: James Yonan <james@openvpn.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>