datapath-windows: Don't leak NBLs with multiple NBs.

Currently, if we receive an NBL with multiple NBs from NDIS, we just
ASSERT() and not do anything. The right thing to do obviously is to
process the NBL. This is a work in progress.

In the meantime, we should complete the NBL and not just leak it.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Tested-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>