3246 ZFS I/O deadman thread

Reviewed by: Matt Ahrens <matthew.ahrens@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Reviewed by: Christopher Siden <chris.siden@delphix.com>
Approved by: Garrett D'Amore <garrett@damore.org>

NOTES: This patch has been reworked from the original in the
following ways to accomidate Linux ZFS implementation

*) Usage of the cyclic interface was replaced by the delayed taskq
   interface.  This avoids the need to implement new compatibility
   code and allows us to rely on the existing taskq implementation.

*) An extern for zfs_txg_synctime_ms was added to sys/dsl_pool.h
   because declaring externs in source files as was done in the
   original patch is just plain wrong.

*) Instead of panicing the system when the deadman triggers a
   zevent describing the blocked vdev and the first pending I/O
   is posted.  If the panic behavior is desired Linux provides
   other generic methods to panic the system when threads are
   observed to hang.

*) For reference, to delay zios by 30 seconds for testing you can
   use zinject as follows: 'zinject -d <vdev> -D30 <pool>'

References:
  illumos/illumos-gate@283b84606b
  https://www.illumos.org/issues/3246

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1396

include/sys/spa_impl.h

@@ -227,6 +227,10 @@ struct spa {
 	uint64_t	spa_feat_for_write_obj;	/* required to write to pool */
 	uint64_t	spa_feat_for_read_obj;	/* required to read from pool */
 	uint64_t	spa_feat_desc_obj;	/* Feature descriptions */
+	taskqid_t	spa_deadman_tqid;	/* Task id */
+	uint64_t	spa_deadman_calls;	/* number of deadman calls */
+	uint64_t	spa_sync_starttime;	/* starting time fo spa_sync */
+	uint64_t	spa_deadman_synctime;	/* deadman expiration timer */
 	/*
 	 * spa_refcnt & spa_config_lock must be the last elements
 	 * because refcount_t changes size based on compilation options.