During pool export flush the ARC asynchronously

This also includes removing L2 vdevs asynchronously.

This commit also guarantees that spa_load_guid is unique.

The zpool reguid feature introduced the spa_load_guid, which is a
transient value used for runtime identification purposes in the ARC.
This value is not the same as the spa's persistent pool guid.

However, the value is seeded from spa_generate_load_guid() which
does not check for uniqueness against the spa_load_guid from other
pools.  Although extremely rare, you can end up with two different
pools sharing the same spa_load_guid value! So we guarantee that
the value is always unique and additionally not still in use by an
async arc flush task.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Don Brady <don.brady@klarasystems.com>
Closes #16215

module/zfs/spa_misc.c

@@ -1588,6 +1588,34 @@ spa_generate_guid(spa_t *spa)
 	return (guid);
 }
 
+static boolean_t
+spa_load_guid_exists(uint64_t guid)
+{
+	avl_tree_t *t = &spa_namespace_avl;
+
+	ASSERT(MUTEX_HELD(&spa_namespace_lock));
+
+	for (spa_t *spa = avl_first(t); spa != NULL; spa = AVL_NEXT(t, spa)) {
+		if (spa_load_guid(spa) == guid)
+			return (B_TRUE);
+	}
+
+	return (arc_async_flush_guid_inuse(guid));
+}
+
+uint64_t
+spa_generate_load_guid(void)
+{
+	uint64_t guid;
+
+	do {
+		(void) random_get_pseudo_bytes((void *)&guid,
+		    sizeof (guid));
+	} while (guid == 0 || spa_load_guid_exists(guid));
+
+	return (guid);
+}
+
 void
 snprintf_blkptr(char *buf, size_t buflen, const blkptr_t *bp)
 {