file: fix numjobs > 1 and implied jobname as filename

file: fix numjobs > 1 and implied jobname as filename

If we have a jobfile that looks like:

[global]
numjobs=4

[/dev/somedevice]

Then we fail jobs 2 and on, since we don't properly add those
files. Fix this by checking if we're generating a filename
based on the jobname.

Fixes: bcbfeefa7bce ("fio: add multi directory support")
Signed-off-by: Jens Axboe <axboe@fb.com>