Yield periodically when rebuilding L2ARC

L2ARC devices of several terabytes filled with 4KB blocks may take 15
minutes to rebuild.  Due to the way L2ARC log reading is implemented
it is quite likely that for all that time rebuild thread will never
sleep.  At least on FreeBSD kernel threads have absolute priority and
can not be preempted by threads with lower priorities.  If some thread
is also bound to that specific CPU it may not get any CPU time for all
the 15 minutes.

Reviewed-by: Cedric Berger <cedric@precidata.com>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Closes #11116

module/zfs/arc.c

@@ -9757,6 +9757,7 @@ l2arc_rebuild(l2arc_dev_t *dev)
 		    !dev->l2ad_first)
 			goto out;
 
+		cond_resched();
 		for (;;) {
 			mutex_enter(&l2arc_rebuild_thr_lock);
 			if (dev->l2ad_rebuild_cancel) {