NVMe: Adhere to request queue block accounting enable/disable

Recently, a new sysfs control "iostats" was added to selectively
enable or disable io statistics collection for request queues.  This
patch hooks that control.

IO statistics collection is rather expensive on large, multi-node
machines with drives pushing millions of iops.  Having the ability to
disable collection if not needed can improve throughput significantly.

As a data point, on a quad E5-4640, I see more than 50% throughput
improvement when io statistics accounting is disabled during heavily
multi-threaded small block random read benchmarks where device
performance is in the million iops+ range.

Signed-off-by: Sam Bradshaw <sbradshaw@micron.com>
Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>