Use smaller default slack/delta value for schedule_hrtimeout_range()

For interrupt coalescing, cv_timedwait_hires() uses a 100us slack/delta
for calls to schedule_hrtimeout_range(). This 100us slack can be costly
for small writes.

This change improves small write performance by passing resolution `res`
parameter to schedule_hrtimeout_range() to be used as delta/slack. A new
tunable `spl_schedule_hrtimeout_slack_us` is added to preserve old
behavior when desired.

Performance observations on 8K recordsize filesystem:
- 8K random writes at 1-64 threads, up to 60% improvement for one thread
  and smaller gains as thread count increases. At >64 threads, 2-5%
  decrease in performance was observed.
- 8K sequential writes, similar 60% improvement for one thread and
  leveling out around 64 threads. At >64 threads, 5-10% decrease in
  performance was observed.
- 128K sequential write sees 1-5 for the 128K. No observed regression at
  high thread count.

Testing done on Ubuntu 18.04 with 4.15 kernel, 8vCPUs and SSD storage on
VMware ESX.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Signed-off-by: Tony Nguyen <tony.nguyen@delphix.com>
Closes #9217

module/zfs/mmp.c

@@ -672,7 +672,7 @@ mmp_thread(void *arg)
 
 		CALLB_CPR_SAFE_BEGIN(&cpr);
 		(void) cv_timedwait_sig_hires(&mmp->mmp_thread_cv,
-		    &mmp->mmp_thread_lock, next_time, USEC2NSEC(1),
+		    &mmp->mmp_thread_lock, next_time, USEC2NSEC(100),
 		    CALLOUT_FLAG_ABSOLUTE);
 		CALLB_CPR_SAFE_END(&cpr, &mmp->mmp_thread_lock);
 	}