Temporary fix for seek, until replaced

I know that Writer is deprecated, but I needed it for something, and the fix is
trivial (and the old code was obviously broken) so I made a fix until
the new IO code is finished.
auto merge of #8098 : jensnockert/rust/fix-fd-seek, r=pcwalton

I know that Writer is deprecated, but I needed it for something, and the fix is trivial (and the old code was obviously broken) so I made a fix until the new IO code is finished.