ext4: Add unit test for mb_free_blocks

Add unit test for mb_free_blocks.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://lore.kernel.org/r/20240103104900.464789-5-shikemeng@huaweicloud.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>