Allocate zap_attribute_t from kmem instead of stack

This patch is preparatory work for long name feature. It changes all
users of zap_attribute_t to allocate it from kmem instead of stack. It
also make zap_attribute_t and zap_name_t structure variable length.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Chunwei Chen <david.chen@nutanix.com>
Closes #15921

module/zfs/spa_misc.c

@@ -2590,6 +2590,7 @@ spa_init(spa_mode_t mode)
 	scan_init();
 	qat_init();
 	spa_import_progress_init();
+	zap_init();
 }
 
 void
@@ -2616,6 +2617,7 @@ spa_fini(void)
 	scan_fini();
 	qat_fini();
 	spa_import_progress_destroy();
+	zap_fini();
 
 	avl_destroy(&spa_namespace_avl);
 	avl_destroy(&spa_spare_avl);