filesetup: don't track file allocation for jobs == 1

filesetup: don't track file allocation for jobs == 1

We only really need this for multiple jobs, and we only check it
if we are in job > 1. But we always mark the file as allocated.
For jobs with lots of files (millions), this takes a long time
to do at setup time.

Running this sample job:

./fio --alloc-size=131072 --name=filecreate --ioengine=filecreate --size=1g --nr_files=1m --openfiles=256

takes ~16s before, and about ~7s afterwards.

Signed-off-by: Jens Axboe <axboe@kernel.dk>