btrfs/172,206: call _log_writes_cleanup in _cleanup

btrfs/172,206: call _log_writes_cleanup in _cleanup

Because block group tree requires require no-holes feature,
_log_writes_mkfs "-O ^no-holes" fails when "-O block-group-tree" is
given in MKFS_OPTION.
Without explicit _log_writes_cleanup, the two tests fail with
logwrites-test device left. And all next tests will fail due to
SCRATCH DEVICE EBUSY.

Fix it by overriding _cleanup to call _log_writes_cleanup.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Su Yue <glass.su@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>