This can only happen on 32-bit systems when you're reading
an uncompressed archive and the skip request is an exact
multiple of 4G (e.g., skipping a tar entry with an 8G body).
The symptom is that the read_ahead() ends up returning zero
bytes, and the extraction stops with a premature end-of-file.
Using '1' here is more correct anyway, as it allows read_ahead()
to function opportunistically and minimize copying.
MFC after: 5 days
7fc8683d7c