ext4: initialize sbi->s_freeclusters_counter and sbi->s_dirtyclusters_counter before use in kunit test

Fix that sbi->s_freeclusters_counter and sbi->s_dirtyclusters_counter are
used before initialization.
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/20240304163543.6700-4-shikemeng@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>