MFC r358580: Increase number of write completion threads, matching ZoL.

Our iSCSI benchmarks on a large 80-core system show that previous limit
of 8 threads can be a bottleneck. At some points this change increases
write IOPS by as much as 50%. I am still not sure that so many threads
is really required, but we tested lower amounts and got no significant
benefits, while latencies were a bit worse, so decided to not diverge.