[ofw] winverbs ND provider

[Sean Hefty]

>Based on ndconn results, the winverbs ND connection rate is still less
>than that reported for the ibal ND provider.

Here is one set of test results.  This is between 2 2-processor systems
with mthca.

winverbs
threads	1	2	4
conn/sec	644	1294	1661
accept	1363	1228	1566
disc		814	715	706

ibal
threads	1	2	4
conn/sec	1454	1854	1823
accept	386	536	568
disc		457	596	772

So, at 4 threads, there's only about a 10% difference, but over a 100%
difference for a single thread.  rdma_cmatose, which is single threaded,
reports about 715 connections / second for winverbs.

I'll try to see what the impact is of using work threads in the kernel
driver.

- Sean