Fix offset retrieval problem for read and write having different bs ranges

Fix offset retrieval problem for read and write having different bs ranges

"Zhang, Yanmin" <yanmin_zhang@linux.intel.com> reported that fio got
stuck with a job file that had different read and write block size
ranges. The problem is that get_next_offset() didn't take the data
direction into account, so when it generated an offset close to the
end of the file, it may not have been legal for that data direction.

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