Fix queue depth for nda.

1/4 of the number of queues times queue entries is too limiting. It
works up to about 4k IOPS / 3.0GB/s for hardware that can do
4.4k/3.2GB/s with nvd. 3/4 works better, though it highlights issues
in the fairness of nda's choice of TRIM vs READ. That will be fixed
separately.