ZTS: checkpoint_discard_busy should use save_tunable/restore_tunable

Instead of using hardwired value for SPA_DISCARD_MEMORY_LIMIT,
use save_tunable and restore_tunable to restore the pre-test state.

Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Signed-off-by: Toomas Soome <[email protected]>
Closes #16919

Author: tsoome

tests/zfs-tests/tests/functional/pool_checkpoint/checkpoint_discard_busy.ksh

@@ -42,8 +42,8 @@ log_unsupported "Skipping, issue https://github.com/openzfs/zfs/issues/12053"
 
 function test_cleanup
 {
-	# reset memory limit to 16M
-	set_tunable64 SPA_DISCARD_MEMORY_LIMIT 16777216
+	# reset to original value
+	log_must restore_tunable SPA_DISCARD_MEMORY_LIMIT
 	cleanup_nested_pools
 }
 
@@ -69,6 +69,7 @@ log_onexit test_cleanup
 #	map, we should have even more time to
 #	verify this.
 #
+log_must save_tunable SPA_DISCARD_MEMORY_LIMIT
 set_tunable64 SPA_DISCARD_MEMORY_LIMIT 128
 
 log_must zpool checkpoint $NESTEDPOOL
@@ -101,8 +102,8 @@ log_mustnot zpool checkpoint -d $NESTEDPOOL
 log_mustnot zpool remove $NESTEDPOOL $FILEDISK1
 log_mustnot zpool reguid $NESTEDPOOL
 
-# reset memory limit to 16M
-set_tunable64 SPA_DISCARD_MEMORY_LIMIT 16777216
+# reset to original value
+log_must restore_tunable SPA_DISCARD_MEMORY_LIMIT
 
 nested_wait_discard_finish