os: fix race between file I/O and Close

os: fix race between file I/O and Close

Now that the os package uses internal/poll on Unix and Windows systems,
it can rely on internal/poll reference counting to ensure that the
file descriptor is not closed until all I/O is complete.

That was already working. This CL completes the job by not trying to
modify the Sysfd field when it might still be used by the I/O routines.

Fixes #7970

Change-Id: I7a3daa1a6b07b7345bdce6f0cd7164bd4eaee952
Reviewed-on: https://go-review.googlesource.com/41674
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>