Revert "Always set ->real_file_size to -1 when failed to get file size"

Revert "Always set ->real_file_size to -1 when failed to get file size"

This reverts commit 34c9f82301a584899a9e65e4b9f56d98c38a52e7.

It breaks a very basic use case, ala:

./fio --ioengine=libaio --rw=randread --numjobs=1 --iodepth=256 --bs=1M --size=10G --direct=1 --filename=/tmp/test100 --name=test

if /tmp/test100 doesn't already exist.