netlink: simplify NLMSG_DATA with NLMSG_HDRLEN

The NLMSG_LENGTH(0) may confuse the API users,
NLMSG_HDRLEN is much more clear.

Besides, some code style problems are also fixed.
Signed-off-by: Chen Li <chenli@uniontech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/netlink.h

@@ -91,9 +91,10 @@ struct nlmsghdr {
 #define NLMSG_HDRLEN	 ((int) NLMSG_ALIGN(sizeof(struct nlmsghdr)))
 #define NLMSG_LENGTH(len) ((len) + NLMSG_HDRLEN)
 #define NLMSG_SPACE(len) NLMSG_ALIGN(NLMSG_LENGTH(len))
-#define NLMSG_DATA(nlh)  ((void*)(((char*)nlh) + NLMSG_LENGTH(0)))
+#define NLMSG_DATA(nlh)  ((void *)(((char *)nlh) + NLMSG_HDRLEN))
 #define NLMSG_NEXT(nlh,len)	 ((len) -= NLMSG_ALIGN((nlh)->nlmsg_len), \
-				  (struct nlmsghdr*)(((char*)(nlh)) + NLMSG_ALIGN((nlh)->nlmsg_len)))
+				  (struct nlmsghdr *)(((char *)(nlh)) + \
+				  NLMSG_ALIGN((nlh)->nlmsg_len)))
 #define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
 			   (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
 			   (nlh)->nlmsg_len <= (len))