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>

include/net/netfilter/nf_nat.h

@@ -104,7 +104,7 @@ unsigned int
 nf_nat_inet_fn(void *priv, struct sk_buff *skb,
 	       const struct nf_hook_state *state);
 
-static inline int nf_nat_initialized(struct nf_conn *ct,
+static inline int nf_nat_initialized(const struct nf_conn *ct,
 				     enum nf_nat_manip_type manip)
 {
 	if (manip == NF_NAT_MANIP_SRC)