Fix hung z_zvol tasks during 'zfs receive'

During a receive operation zvol_create_minors_impl() can wait
needlessly for the prefetch thread because both share the same tasks
queue.  This results in hung tasks:

<3>INFO: task z_zvol:5541 blocked for more than 120 seconds.
<3>      Tainted: P           O  3.16.0-4-amd64
<3>"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

The first z_zvol:5541 (zvol_task_cb) is waiting for the long running
traverse_prefetch_thread:260

root@linux:~# cat /proc/spl/taskq
taskq                       act  nthr  spwn  maxt   pri  mina
spl_system_taskq/0            1     2     0    64   100     1
	active: [260]traverse_prefetch_thread [zfs](0xffff88003347ae40)
	wait: 5541
spl_delay_taskq/0             0     1     0     4   100     1
	delay: spa_deadman [zfs](0xffff880039924000)
z_zvol/1                      1     1     0     1   120     1
	active: [5541]zvol_task_cb [zfs](0xffff88001fde6400)
	pend: zvol_task_cb [zfs](0xffff88001fde6800)

This change adds a dedicated, per-pool, prefetch taskq to prevent the
traverse code from monopolizing the global (and limited) system_taskq by
inappropriately scheduling long running tasks on it.

Reviewed-by: Albert Lee <trisk@forkgnu.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6330
Closes #6890
Closes #7343

include/sys/spa_impl.h

@@ -275,6 +275,7 @@ struct spa {
 	spa_stats_t	spa_stats;		/* assorted spa statistics */
 	hrtime_t	spa_ccw_fail_time;	/* Conf cache write fail time */
 	taskq_t		*spa_zvol_taskq;	/* Taskq for minor management */
+	taskq_t		*spa_prefetch_taskq;	/* Taskq for prefetch threads */
 	uint64_t	spa_multihost;		/* multihost aware (mmp) */
 	mmp_thread_t	spa_mmp;		/* multihost mmp thread */