dm: don't pass bio to __dm_start_io_acct and dm_end_io_acct

dm->orig_bio is always passed to __dm_start_io_acct and dm_end_io_acct,
so it isn't necessary to take one bio parameter for the two helpers.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>