Parallel pool import

This commit allow spa_load() to drop the spa_namespace_lock so
that imports can happen concurrently. Prior to dropping the
spa_namespace_lock, the import logic will set the spa_load_thread
value to track the thread which is doing the import.

Consumers of spa_lookup() retain the same behavior by blocking
when either a thread is holding the spa_namespace_lock or the
spa_load_thread value is set. This will ensure that critical
concurrent operations cannot take place while a pool is being
imported.

The zpool command is also enhanced to provide multi-threaded support
when invoking zpool import -a.

Lastly, zinject provides a mechanism to insert artificial delays
when importing a pool and new zfs tests are added to verify parallel
import functionality.

Contributions-by: Don Brady <don.brady@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Wilson <gwilson@delphix.com>
Closes #16093

module/zfs/vdev_rebuild.c

@@ -23,6 +23,7 @@
  * Copyright (c) 2018, Intel Corporation.
  * Copyright (c) 2020 by Lawrence Livermore National Security, LLC.
  * Copyright (c) 2022 Hewlett Packard Enterprise Development LP.
+ * Copyright (c) 2024 by Delphix. All rights reserved.
  */
 
 #include <sys/vdev_impl.h>
@@ -1071,7 +1072,8 @@ vdev_rebuild_restart_impl(vdev_t *vd)
 void
 vdev_rebuild_restart(spa_t *spa)
 {
-	ASSERT(MUTEX_HELD(&spa_namespace_lock));
+	ASSERT(MUTEX_HELD(&spa_namespace_lock) ||
+	    spa->spa_load_thread == curthread);
 
 	vdev_rebuild_restart_impl(spa->spa_root_vdev);
 }