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>