datapath-windows: Incorrect assumption of the IRQL

Acquiring a spin lock raises the IRQL to DISPATCH_LEVEL. But
in many places of the code, while holding a spin lock, there
are useless checks for the current IRQL against DISPATCH_LEVEL.
Also, the dispatch flag is not correctly set when calling
NdisAcquireRWLockXXX() functions, which causes an extra check
of the current IRQL.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Reported-at: https://github.com/openvswitch/ovs-issues/issues/47
Acked-by: Nithin Raju <nithin@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>