add do_support_discard module parameter.
walb does not support write same explicitly.
fix: do not use bio_clone() to clone deeply.
add bio_calc_checksum_iter().
bugfix: bio contents may change during IO execution.

We avoid checksum invalidation issue by copying bio contents at first and
use its copy ever after so that walb code will see a fixed image.

We observed ext4 code seems to change meta data
after submission of the corresponding write IOs.