Handle short read at the end of file

Currently we don't issue an io operation if offset + buflen
is larger than the file size, but that means we don't reach
the last bit of a file if the size isn't block size aligned.
Instead handle a short read properly. It actually cleans up
the code as well.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>