Implemented zpool scrub pause/resume

Currently, there is no way to pause a scrub. Pausing may
be useful when the pool is busy with other I/O to preserve
bandwidth.

This patch adds the ability to pause and resume scrubbing.
This is achieved by maintaining a persistent on-disk scrub state.
While the state is 'paused' we do not scrub any more blocks.
We do however perform regular scan housekeeping such as
freeing async destroyed and deadlist blocks while paused.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Thomas Caputi <tcaputi@datto.com>
Reviewed-by: Serapheim Dimitropoulos <serapheimd@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alek Pinchuk <apinchuk@datto.com>
Closes #6167

include/sys/spa_impl.h

@@ -25,6 +25,7 @@
  * Copyright (c) 2014 Spectra Logic Corporation, All rights reserved.
  * Copyright 2013 Saso Kiselkov. All rights reserved.
  * Copyright (c) 2016 Actifio, Inc. All rights reserved.
+ * Copyright (c) 2017 Datto Inc.
  */
 
 #ifndef _SYS_SPA_IMPL_H
@@ -193,6 +194,8 @@ struct spa {
 	uint8_t		spa_scrub_started;	/* started since last boot */
 	uint8_t		spa_scrub_reopen;	/* scrub doing vdev_reopen */
 	uint64_t	spa_scan_pass_start;	/* start time per pass/reboot */
+	uint64_t	spa_scan_pass_scrub_pause; /* scrub pause time */
+	uint64_t	spa_scan_pass_scrub_spent_paused; /* total paused */
 	uint64_t	spa_scan_pass_exam;	/* examined bytes per pass */
 	kmutex_t	spa_async_lock;		/* protect async state */
 	kthread_t	*spa_async_thread;	/* thread doing async task */