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

In some cases it's set to -1, but on others it's not,
while the existing code (e.g. get_fs_free_counts(), setup_files(), etc)
seem to expect ->real_file_size to be -1 in case of any error.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>