syscall: store skip count in file descriptor offset

Multiple calls to ReadDirent expect to return subsequent
portions of the directory listing. There's no place to store
our progress other than the file descriptor offset.

Fortunately, the file descriptor offset doesn't need to be
a real offset. We can store any int64 we want there.

Fixes #31368

Change-Id: I49e4e0e7ff707d3e96aa5d43e3b0199531013cde
Reviewed-on: https://go-review.googlesource.com/c/go/+/171477


Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>