netfilter: conntrack: use correct format characters

When compiling with -Wformat, clang emits the following warnings:

net/netfilter/nf_conntrack_helper.c:168:18: error: format string is not a string literal (potentially insecure) [-Werror,-Wformat-security]
                request_module(mod_name);
                               ^~~~~~~~

Use a string literal for the format string.

Link: https://github.com/ClangBuiltLinux/linux/issues/378
Signed-off-by: Bill Wendling <isanbard@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_helper.c

@@ -165,7 +165,7 @@ nf_nat_helper_try_module_get(const char *name, u16 l3num, u8 protonum)
 	if (!nat) {
 		snprintf(mod_name, sizeof(mod_name), "%s", h->nat_mod_name);
 		rcu_read_unlock();
-		request_module(mod_name);
+		request_module("%s", mod_name);
 
 		rcu_read_lock();
 		nat = nf_conntrack_nat_helper_find(mod_name);