Fix MMP write frequency for large pools

When a single pool contains more vdevs than the CONFIG_HZ for
for the kernel the mmp thread will not delay properly.  Switch
to using cv_timedwait_sig_hires() to handle higher resolution
delays.

This issue was reported on Arch Linux where HZ defaults to only
100 and this could be fairly easily reproduced with a reasonably
large pool.  Most distribution kernels set CONFIG_HZ=250 or
CONFIG_HZ=1000 and thus are unlikely to be impacted.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7205 
Closes #7289

module/zfs/mmp.c

@@ -527,9 +527,9 @@ mmp_thread(void *arg)
 			mmp_write_uberblock(spa);
 
 		CALLB_CPR_SAFE_BEGIN(&cpr);
-		(void) cv_timedwait_sig(&mmp->mmp_thread_cv,
-		    &mmp->mmp_thread_lock, ddi_get_lbolt() +
-		    ((next_time - gethrtime()) / (NANOSEC / hz)));
+		(void) cv_timedwait_sig_hires(&mmp->mmp_thread_cv,
+		    &mmp->mmp_thread_lock, next_time, USEC2NSEC(1),
+		    CALLOUT_FLAG_ABSOLUTE);
 		CALLB_CPR_SAFE_END(&cpr, &mmp->mmp_thread_lock);
 	}