os: only employ sendfile(3ext) on illumos when target is regular file
Fixes #68863
Change-Id: I0ca324137d1f7510bd0b245791fef07d3b5b401a
Reviewed-on: https://go-review.googlesource.com/c/go/+/605355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Run-TryBot: Andy Pan <panjf2000@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>