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/vdev_rebuild.c

@@ -1079,7 +1079,7 @@ vdev_rebuild_restart_impl(vdev_t *vd)
 void
 vdev_rebuild_restart(spa_t *spa)
 {
-	ASSERT(MUTEX_HELD(&spa_namespace_lock) ||
+	ASSERT(spa_namespace_held() ||
 	    spa->spa_load_thread == curthread);
 
 	vdev_rebuild_restart_impl(spa->spa_root_vdev);
@@ -1094,7 +1094,7 @@ vdev_rebuild_stop_wait(vdev_t *vd)
 {
 	spa_t *spa = vd->vdev_spa;
 
-	ASSERT(MUTEX_HELD(&spa_namespace_lock) ||
+	ASSERT(spa_namespace_held() ||
 	    spa->spa_export_thread == curthread);
 
 	if (vd == spa->spa_root_vdev) {