libnetlink: Use NLMSG_LENGTH to set nlmsg_len

libnetlink: Use NLMSG_LENGTH to set nlmsg_len

Change nlmsg_len from sizeof(req) to use NLMSG_LENGTH on the header.
2 of the inner headers are not 4-byte aligned, so add a 0-length buf
after the header with the __aligned(NLMSG_ALIGNTO) to ensure the size
of the request is large enough. Use NLMSG_ALIGN in NLMSG_LENGTH to set
nlmsg_len.

Signed-off-by: David Ahern <dsahern@gmail.com>