Ratelimit deadman zevents as with delay zevents

Just as delay zevents can flood the zevent pipe when a vdev becomes
unresponsive, so do the deadman zevents.

Ratelimit deadman zevents according to the same tunable as for delay
zevents.

Enable deadman tests on FreeBSD and add a test for deadman event
ratelimiting. 

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Don Brady <don.brady@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11786

module/zfs/vdev.c

@@ -625,6 +625,8 @@ vdev_alloc_common(spa_t *spa, uint_t id, uint64_t guid, vdev_ops_t *ops)
 	 */
 	zfs_ratelimit_init(&vd->vdev_delay_rl, &zfs_slow_io_events_per_second,
 	    1);
+	zfs_ratelimit_init(&vd->vdev_deadman_rl, &zfs_slow_io_events_per_second,
+	    1);
 	zfs_ratelimit_init(&vd->vdev_checksum_rl,
 	    &zfs_checksum_events_per_second, 1);
 
@@ -1106,6 +1108,7 @@ vdev_free(vdev_t *vd)
 	cv_destroy(&vd->vdev_rebuild_cv);
 
 	zfs_ratelimit_fini(&vd->vdev_delay_rl);
+	zfs_ratelimit_fini(&vd->vdev_deadman_rl);
 	zfs_ratelimit_fini(&vd->vdev_checksum_rl);
 
 	if (vd == spa->spa_root_vdev)

module/zfs/zfs_fm.c

@@ -395,8 +395,8 @@ zfs_zevent_post_cb(nvlist_t *nvl, nvlist_t *detector)
 }
 
 /*
- * We want to rate limit ZIO delay and checksum events so as to not
- * flood ZED when a disk is acting up.
+ * We want to rate limit ZIO delay, deadman, and checksum events so as to not
+ * flood zevent consumers when a disk is acting up.
  *
  * Returns 1 if we're ratelimiting, 0 if not.
  */
@@ -405,11 +405,13 @@ zfs_is_ratelimiting_event(const char *subclass, vdev_t *vd)
 {
 	int rc = 0;
 	/*
-	 * __ratelimit() returns 1 if we're *not* ratelimiting and 0 if we
+	 * zfs_ratelimit() returns 1 if we're *not* ratelimiting and 0 if we
 	 * are.  Invert it to get our return value.
 	 */
 	if (strcmp(subclass, FM_EREPORT_ZFS_DELAY) == 0) {
 		rc = !zfs_ratelimit(&vd->vdev_delay_rl);
+	} else if (strcmp(subclass, FM_EREPORT_ZFS_DEADMAN) == 0) {
+		rc = !zfs_ratelimit(&vd->vdev_deadman_rl);
 	} else if (strcmp(subclass, FM_EREPORT_ZFS_CHECKSUM) == 0) {
 		rc = !zfs_ratelimit(&vd->vdev_checksum_rl);
 	}