netfilter: Use LOGLEVEL_<FOO> defines

Use the #defines where appropriate.

Miscellanea:

Add explicit #include <linux/kernel.h> where it was not
previously used so that these #defines are a bit more
explicitly defined instead of indirectly included via:
	module.h->moduleparam.h->kernel.h

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/net/netfilter/nft_log.c b/net/netfilter/nft_log.c
index bde05f28..e18af9db 100644
--- a/net/netfilter/nft_log.c
+++ b/net/netfilter/nft_log.c
@@ -78,7 +78,7 @@
 			li->u.log.level =
 				ntohl(nla_get_be32(tb[NFTA_LOG_LEVEL]));
 		} else {
-			li->u.log.level = 4;
+			li->u.log.level = LOGLEVEL_WARNING;
 		}
 		if (tb[NFTA_LOG_FLAGS] != NULL) {
 			li->u.log.logflags =