ipv4: coding style: comparison for equality with NULL

The ipv4 code uses a mixture of coding styles. In some instances check
for NULL pointer is done as x == NULL and sometimes as !x. !x is
preferred according to checkpatch and this patch makes the code
consistent by adopting the latter form.

No changes detected by objdiff.

Signed-off-by: Ian Morris <ipm@chirality.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index c224c85..bff6974 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -298,7 +298,7 @@
 {
 	struct ctl_table *table;
 
-	if (net->ipv4.xfrm4_hdr == NULL)
+	if (!net->ipv4.xfrm4_hdr)
 		return;
 
 	table = net->ipv4.xfrm4_hdr->ctl_table_arg;