Don't malloc more than necessary on extending/prereading file

If td->o.max_bs[DDIR_XXX] were larger than file size to extend/preread,
it only needs to malloc the file size.

Given that td->o.max_bs[DDIR_XXX] are tunable parameters, it's better
not to malloc whatever size specified if needed size is smaller than
that. Users could specify as large as maximum of uint.

I dropped "bs = td->o.max_bs[DDIR_WRITE];" inside the first while loop
since now it works the same with or without this.

(This commit directly goes on top of the previous one)

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>