datapath-windows/NetlinkBuf.h: Added NlBufSize

Added an inline function to return used size in the buffer.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Acked-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>