ext4: move checksum length calculation of inode bitmap into ext4_inode_bitmap_csum_[verify/set]() functions

There are some little improve:
1. remove repeat code to calculate checksum length of inode bitmap
2. remove unnecessary checksum length calculation if checksum is not
enabled.
3. use more efficient bit shift operation instead of div opreation.
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Link: https://patch.msgid.link/20240820132234.2759926-6-shikemeng@huaweicloud.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>