fseeko: fix bug on glibc

fseeko: fix bug on glibc

Commit 1074f45959f had a bug on glibc that caused the unit test to
fail, but instead of figuring why the failure was happening, we
instead added a partial hack to avoid the problem in the scenario
covered by the testsuite.
https://lists.gnu.org/archive/html/bug-gnulib/2009-01/msg00171.html

But that hack doesn't cover all scenarios, and papers over the
real issue: if you fopen()/ftell()/fseek(,0,SEEK_END)/ftell(),
then the stream should be positioned at the same place as the
underlying fd, even though no I/O occurred, and without requiring
an fflush() in the middle.

* lib/fseeko.c (fseeko): Set stream offset to match fd offset.
Reported by John W. Eaton.

Signed-off-by: Eric Blake <eblake@redhat.com>