net: introduce helper macro for_each_cmsghdr

Introduce helper macro for_each_cmsghdr as a wrapper of the enumerating
cmsghdr from msghdr, just cleanup.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/crypto/af_alg.c b/crypto/af_alg.c
index 6a3ad80..bc21f52 100644
--- a/crypto/af_alg.c
+++ b/crypto/af_alg.c
@@ -399,7 +399,7 @@
 {
 	struct cmsghdr *cmsg;
 
-	for (cmsg = CMSG_FIRSTHDR(msg); cmsg; cmsg = CMSG_NXTHDR(msg, cmsg)) {
+	for_each_cmsghdr(cmsg, msg) {
 		if (!CMSG_OK(msg, cmsg))
 			return -EINVAL;
 		if (cmsg->cmsg_level != SOL_ALG)