jfs: Regular file corruption check

The reproducer builds a corrupted file on disk with a negative i_size value.
Add a check when opening this file to avoid subsequent operation failures.

Reported-by:  <syzbot+630f6d40b3ccabc8e96e@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=630f6d40b3ccabc8e96e


Tested-by:  <syzbot+630f6d40b3ccabc8e96e@syzkaller.appspotmail.com>
Signed-off-by: Edward Adam Davis <eadavis@qq.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>