netlink: remove duplicated NLMSG_ALIGN

NLMSG_HDRLEN is already aligned value. It's for directly reference
without extra alignment.

The redundant alignment here may confuse the API users.

Signed-off-by: Hong Zhiguo <honkiko@gmail.com>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>