zfs: lower priority of zio_write_issue threads by four

The difference of one was insignificant because zio_write_issue threads
ended up on the same run queues as other zio threads.
See sys/priority.h and sys/runq.h for more details.

Add a comment describing FreeBSD priority considerations and restore
the illumos variant of the code for comparison.

Obtained from:	Panzura
MFC after:	2 weeks
Sponsored by:	Panzura

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/spa.c

@@ -922,9 +922,17 @@ spa_taskqs_init(spa_t *spa, zio_type_t t, zio_taskq_type_t q)
 			 * The write issue taskq can be extremely CPU
 			 * intensive.  Run it at slightly lower priority
 			 * than the other taskqs.
+			 * FreeBSD notes:
+			 * - numerically higher priorities are lower priorities;
+			 * - if priorities divided by four (RQ_PPQ) are equal
+			 *   then a difference between them is insignificant.
 			 */
 			if (t == ZIO_TYPE_WRITE && q == ZIO_TASKQ_ISSUE)
-				pri++;
+#ifdef illumos
+				pri--;
+#else
+				pri += 4;
+#endif
 
 			tq = taskq_create_proc(name, value, pri, 50,
 			    INT_MAX, spa->spa_proc, flags);