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/zfs_ioctl.h

@@ -236,6 +236,8 @@ typedef struct zinject_record {
 	uint32_t	zi_iotype;
 	int32_t		zi_duration;
 	uint64_t	zi_timer;
+	uint32_t	zi_cmd;
+	uint32_t	zi_pad;
 } zinject_record_t;
 
 #define	ZINJECT_NULL		0x1
@@ -245,6 +247,16 @@ typedef struct zinject_record {
 #define	ZEVENT_NONBLOCK		0x1
 #define	ZEVENT_SIZE		1024
 
+typedef enum zinject_type {
+	ZINJECT_UNINITIALIZED,
+	ZINJECT_DATA_FAULT,
+	ZINJECT_DEVICE_FAULT,
+	ZINJECT_LABEL_FAULT,
+	ZINJECT_IGNORED_WRITES,
+	ZINJECT_PANIC,
+	ZINJECT_DELAY_IO,
+} zinject_type_t;
+
 typedef struct zfs_share {
 	uint64_t	z_exportdata;
 	uint64_t	z_sharedata;