vdev probe to slow disk can stall mmp write checker

Simplify vdev probes in the zio_vdev_io_done context to
avoid holding the spa config lock for a long duration.

Also allow zpool clear if no evidence of another host
is using the pool.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #15839

module/zfs/txg.c

@@ -550,6 +550,15 @@ txg_sync_thread(void *arg)
 			timer = (delta > timeout ? 0 : timeout - delta);
 		}
 
+		/*
+		 * When we're suspended, nothing should be changing and for
+		 * MMP we don't want to bump anything that would make it
+		 * harder to detect if another host is changing it when
+		 * resuming after a MMP suspend.
+		 */
+		if (spa_suspended(spa))
+			continue;
+
 		/*
 		 * Wait until the quiesce thread hands off a txg to us,
 		 * prompting it to do so if necessary.