Enable read/write sequential offset to be a non-multiple of the block size

Enable read/write sequential offset to be a non-multiple of the block size

The way that fio does offset math internally, if a non-multiple
of the blocksize was given as a sequence offset, it would simply
be ignored due to the block offset division rounding it down.
In other words, if you did:

rw=read:512k
bs=1m

it did not read 1m and 0 offset, 1m at 512k offset, etc. Instead
the hole is ignored.

Fix this by leaving the generated sequential offset in bytes
instead of turning it into a 'fio block'.

Signed-off-by: Jens Axboe <axboe@kernel.dk>