blktrace: Reorganize creation of output file name

We would like to generate output file name without having corresponding
iop structure. Reorganize the function to allow that. Also fix couple of
overflows possible when generating the file name when we are modifying
the code anyway.

Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>