ipv6: coding style: comparison for equality with NULL

The ipv6 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/ipv6/xfrm6_policy.c b/net/ipv6/xfrm6_policy.c
index 11dbcc1..91d934c 100644
--- a/net/ipv6/xfrm6_policy.c
+++ b/net/ipv6/xfrm6_policy.c
@@ -370,7 +370,7 @@
 {
 	struct ctl_table *table;
 
-	if (net->ipv6.sysctl.xfrm6_hdr == NULL)
+	if (!net->ipv6.sysctl.xfrm6_hdr)
 		return;
 
 	table = net->ipv6.sysctl.xfrm6_hdr->ctl_table_arg;