ZTS: checkpoint_discard_busy should use save_tunable/restore_tunable

[tsoome]

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

Motivation and Context

Description

Once tests are done, we want to restore system state to the pre-tests state, which may be different from currently hardwired value.

How Has This Been Tested?

Types of changes

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• [x ] My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• [x ] All commit messages are properly formatted and contain Signed-off-by.

[amotin]

Thanks. Just a nit that all existing restore_tunable uses do it under log_must. Would be good to add it here too.

[tsoome]

Thanks. Just a nit that all existing restore_tunable uses do it under log_must. Would be good to add it here too.

done.