Add alloc_new_file()

Hopefully avoid a bug like commit 4db205dc fixed by abstracting
out the allocate-and-initialize part of fio_file allocation.

Signed-off-by: Jens Axboe <axboe@fb.com>
File pre-read lseek error check

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