os: only employ sendfile(3ext) on illumos when target is regular file
Follows up CL 605355
Fixes #68863
Change-Id: I56e05822502e66eed610d5e924d110607ce146b5
Reviewed-on: https://go-review.googlesource.com/c/go/+/606135
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
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