datapath-windows: fix bug in NlBufCopyAtTailUninit

We should be returning value of tail before the increment
and not after.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Ankur Sharma <ankursharma@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
datapath-windows: use the Netlink set API and need new APIs

In this change:
1. we refactor the code that fills up information about the DP into
a seprate function.
2. use the netlink set APIs to fill up the netlink attributes.
3. we define a OVS_DP_STATS to be a typedef of 'struct ovs_dp_stats'
in keeping with the Windows kernel naming conventions.
4. In the absence of netlink set API, I had put in an ASSERT earlier
that the output buffer should be limited to 512 bytes. This is not
true anymore. The netlink set API checks for bounds of the buffer.
Hence removed the ASSERT.

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