spa_misc: add an API for spa_namespace_lock

This is useful as debugging support, as it lets namespace lock
operations be traced directly. It will also be useful for future work to
reduce the use of spa_namespace_lock, traditionally a source of
difficult deadlocks.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17906

module/zfs/arc.c

@@ -8548,7 +8548,7 @@ l2arc_dev_get_next(void)
 	 * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
 	 * both locks will be dropped and a spa config lock held instead.
 	 */
-	mutex_enter(&spa_namespace_lock);
+	spa_namespace_enter(FTAG);
 	mutex_enter(&l2arc_dev_mtx);
 
 	/* if there are no vdevs, there is nothing to do */
@@ -8591,7 +8591,7 @@ out:
 	 */
 	if (next != NULL)
 		spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
-	mutex_exit(&spa_namespace_lock);
+	spa_namespace_exit(FTAG);
 
 	return (next);
 }
@@ -10231,7 +10231,7 @@ l2arc_stop(void)
 void
 l2arc_spa_rebuild_start(spa_t *spa)
 {
-	ASSERT(MUTEX_HELD(&spa_namespace_lock));
+	ASSERT(spa_namespace_held());
 
 	/*
 	 * Locate the spa's l2arc devices and kick off rebuild threads.
@@ -10256,7 +10256,7 @@ l2arc_spa_rebuild_start(spa_t *spa)
 void
 l2arc_spa_rebuild_stop(spa_t *spa)
 {
-	ASSERT(MUTEX_HELD(&spa_namespace_lock) ||
+	ASSERT(spa_namespace_held() ||
 	    spa->spa_export_thread == curthread);
 
 	for (int i = 0; i < spa->spa_l2cache.sav_count; i++) {