syscall: Setrlimit: always clean rlimitNofileCache

Since the introduction of origRlimitNofileCache in CL 476097 the only way to
disable restoring RLIMIT_NOFILE before calling execve syscall
(os.StartProcess etc) is this:

	var r syscall.Rlimit
	syscall.Getrlimit(syscall.RLIMIT_NOFILE, &r)
	syscall.Setrlimit(syscall.RLIMIT_NOFILE, &r)

The problem is, this only works when setrlimit syscall succeeds, which
is not possible in some scenarios.

Let's assume that if a user calls syscall.Setrlimit, they
unconditionally want to disable restoring the original rlimit.

For #66797.

Change-Id: I20d0365df4bd6a5c3cc8c22b0c0db87a25b52746
Reviewed-on: https://go-review.googlesource.com/c/go/+/588076
Run-TryBot: Kirill Kolyshkin <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
TryBot-Bypass: Ian Lance Taylor <[email protected]>

src/syscall/rlimit.go

@@ -50,11 +50,10 @@ func init() {
 }
 
 func Setrlimit(resource int, rlim *Rlimit) error {
-	err := setrlimit(resource, rlim)
-	if err == nil && resource == RLIMIT_NOFILE {
+	if resource == RLIMIT_NOFILE {
 		// Store nil in origRlimitNofile to tell StartProcess
 		// to not adjust the rlimit in the child process.
 		origRlimitNofile.Store(nil)
 	}
-	return err
+	return setrlimit(resource, rlim)
 }