OpenZFS 9993 - zil writes can get delayed in zio pipeline

Authored by: George Wilson <george.wilson@delphix.com>
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Brad Lewis <brad.lewis@delphix.com>
Reviewed by: Matt Ahrens <matt@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/9993
OpenZFS-commit: openzfs/openzfs@2258ad0b

module/zfs/zio.c

@@ -1738,7 +1738,8 @@ zio_taskq_dispatch(zio_t *zio, zio_taskq_type_t q, boolean_t cutinline)
 	 * If this is a high priority I/O, then use the high priority taskq if
 	 * available.
 	 */
-	if (zio->io_priority == ZIO_PRIORITY_NOW &&
+	if ((zio->io_priority == ZIO_PRIORITY_NOW ||
+	    zio->io_priority == ZIO_PRIORITY_SYNC_WRITE) &&
 	    spa->spa_zio_taskq[t][q + 1].stqs_count != 0)
 		q++;