ext4: hold group lock in ext4 kunit test

ext4: hold group lock in ext4 kunit test

Although there is no concurrent block allocation/free in unit test,
internal functions mb_mark_used and mb_free_blocks assert group
lock is always held. Acquire group before calling mb_mark_used and
mb_free_blocks in unit test to avoid the assertion.

Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20240304163543.6700-3-shikemeng@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>