filesetup: honor the offset option

filesetup: honor the offset option

Commands like the following do not honor the value given by the offset
option:

./fio --name=test --rw=randread --runtime=10s --offset=90% --time_based --ioengine=null --size=1T --norandommap --randrepeat=0
./fio --name=test --size=8k --offset=4k

In the random case, eventually a random offset will be generated beyond
the 1T file size, leading to a failure.

In the sequential case, a 12k file will be created despite size
specifying the 8k end boundary.

This patch modifies setup_files() so that f->io_size incorporates the
offset for cases like those above.

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