cmd/vet: extra args if any formats are indexed are ok

For example, the following program is valid:

	func main() {
		fmt.Printf("%[1]d", 1, 2, 3)
	}

If any of the formats are indexed, fmt will not complain about unused
extra arguments. See #22867 for more detail.

Make vet follow the same logic, to avoid erroring on programs that would
run without fmt complaining.

Fixes #23564.

Change-Id: Ic9dede5d4c37d1cd4fa24714216944897b5bb7cc
Reviewed-on: https://go-review.googlesource.com/90495


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>