Make bpfmt use exit codes even when reading from stdin.

Previously, an incantation like `bpfmt -o <$BAD_BP` would print to
stderr as expected, but then it would exit with 0. This is different
than the behavior of `bpfmt -o $BAD_BP`, which correctly exits with a
non-zero status code (in addition to printing the issues to stderr).

Test: m bpfmt; out/soong/host/linux-x86/bin/bpfmt -o <$BAD_BP; echo $?
Change-Id: I0a970cddf94fbd818525f9167d9113086b023c81

bpfmt/bpfmt.go

@@ -141,7 +141,7 @@ func main() {
 		if err := processReader("<standard input>", os.Stdin, os.Stdout); err != nil {
 			report(err)
 		}
-		return
+		os.Exit(exitCode)
 	}
 
 	for i := 0; i < flag.NArg(); i++ {