netlink: Fix RTM_SETLINK padding at the end of the message

netlink: Fix RTM_SETLINK padding at the end of the message

While the kernel seems to have accepted the message to set linkmode and
operstate without the final attribute getting padded to 32-bit length,
it is better to get this cleaned up to match expected format. The double
NLMSG_ALIGN() followed by RTA_LENGTH() did not make much sense here.

Signed-off-by: Jouni Malinen <j@w1.fi>