internal/poll: remove fallback path in accept

Support for operating system versions requiring the fallback to
CloseOnExec/SetNonblock was dropped from recent Go versions. The minimum
Linux kernel version is 2.6.32 as of Go 1.18. FreeBSD 10 is no longer
supported as of Go 1.13.

Follows a similar change for net.sysSocket in CL 403634 and
syscall.Socket in CL 422374.

For #45964

Change-Id: I60848415742a1d8204e1fda585462ff35ad6722f
Reviewed-on: https://go-review.googlesource.com/c/go/+/422375
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

Author: tklauser

Diff for: src/internal/poll/sock_cloexec.go

@@ -15,36 +15,8 @@ import "syscall"
 // descriptor as nonblocking and close-on-exec.
 func accept(s int) (int, syscall.Sockaddr, string, error) {
 	ns, sa, err := Accept4Func(s, syscall.SOCK_NONBLOCK|syscall.SOCK_CLOEXEC)
-	// On Linux the accept4 system call was introduced in 2.6.28
-	// kernel and on FreeBSD it was introduced in 10 kernel. If we
-	// get an ENOSYS error on both Linux and FreeBSD, or EINVAL
-	// error on Linux, fall back to using accept.
-	switch err {
-	case nil:
-		return ns, sa, "", nil
-	default: // errors other than the ones listed
-		return -1, sa, "accept4", err
-	case syscall.ENOSYS: // syscall missing
-	case syscall.EINVAL: // some Linux use this instead of ENOSYS
-	case syscall.EACCES: // some Linux use this instead of ENOSYS
-	case syscall.EFAULT: // some Linux use this instead of ENOSYS
-	}
-
-	// See ../syscall/exec_unix.go for description of ForkLock.
-	// It is probably okay to hold the lock across syscall.Accept
-	// because we have put fd.sysfd into non-blocking mode.
-	// However, a call to the File method will put it back into
-	// blocking mode. We can't take that risk, so no use of ForkLock here.
-	ns, sa, err = AcceptFunc(s)
-	if err == nil {
-		syscall.CloseOnExec(ns)
-	}
 	if err != nil {
-		return -1, nil, "accept", err
-	}
-	if err = syscall.SetNonblock(ns, true); err != nil {
-		CloseFunc(ns)
-		return -1, nil, "setnonblock", err
+		return -1, sa, "accept4", err
 	}
 	return ns, sa, "", nil
 }