cmd/gofmt: fix symlink file truncation

Currenty `go fmt` does not truncate the symlinked file, as it compares the size of the recorded data not with the real file size, but with the symlink reference path length.

Fixes #79735

Change-Id: Ic52e8ee4a1c6047a8e3f7d303fc8019beceb910e
GitHub-Last-Rev: 5a37ef7f9ceed60060d11edcca0f8d26f2bc3c3b
GitHub-Pull-Request: golang/go#79715
Reviewed-on: https://go-review.googlesource.com/c/go/+/784320
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>