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/vdev_label.c

@@ -2027,6 +2027,7 @@ retry:
 	/*
 	 * If this isn't a resync due to I/O errors,
 	 * and nothing changed in this transaction group,
+	 * and multihost protection isn't enabled,
 	 * and the vdev configuration hasn't changed,
 	 * then there's nothing to do.
 	 */
@@ -2034,7 +2035,8 @@ retry:
 		boolean_t changed = uberblock_update(ub, spa->spa_root_vdev,
 		    txg, spa->spa_mmp.mmp_delay);
 
-		if (!changed && list_is_empty(&spa->spa_config_dirty_list))
+		if (!changed && list_is_empty(&spa->spa_config_dirty_list) &&
+		    !spa_multihost(spa))
 			return (0);
 	}