Add low water mark for queuing depth

Current fio will attempt to keep the queue full at all times,
but sometimes that's not what you want. Add iodepth_low to
indicate a low water mark for queuing depth, so that when we
see a FIO_Q_BUSY or run out of free requests, let the queue
drain down to the iodepth_low setting before building it up
again.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 38de5e3..282ccf0 100644
--- a/fio.h
+++ b/fio.h
@@ -325,6 +325,7 @@
 	unsigned int stonewall;
 	unsigned int numjobs;
 	unsigned int iodepth;
+	unsigned int iodepth_low;
 	os_cpu_mask_t cpumask;
 	unsigned int iolog;
 	unsigned int read_iolog;