ext4: add test_mb_mark_used_cost to estimate cost of mb_mark_used

Add test_mb_mark_used_cost to estimate cost of mb_mark_used

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240424061904.987525-3-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>