datapath-windows: NetlinkProto: Fix typo and add padding macro.

Added a new macro for calculating the number of bytes required
for padding. Fixed a minor typo.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Tested-by: Ankur Sharma <ankursharma@vmware.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/37
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean at cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>