Before this change, if a file shrunk in size on the remote then rclone
could get into an loop trying to download the file forever.
The symptom was repeating errors like this:
vfs cache: restart download failed: failed to start downloader: failed to open downloader: vfs reader: failed to open source file: invalid seek position
The fix was to check that file size in various places and makes sure
that we weren't trying to download too much data.
This was a problems with backends (like s3) which update the size of
the object on Open to the actual size of the object.