unix: add openat2 for linux

openat2 is a new syscall added to Linux 5.6. It provides a superset
of openat(2) functionality, providing a way to extend it in the future,
and (for now) adding flags telling the kernel how to resolve the path.

For more info on openat2, see https://lwn.net/Articles/803237/

A primitive test case is added to check that Openat2 works as
it should. Tested to skip on kernel 5.5 and pass on 5.6.

Change-Id: Ib8bbd71791762f043200543cecdea16d2fd3c81d
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
Reviewed-on: https://go-review.googlesource.com/c/sys/+/227280
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

unix/syscall_linux_test.go

@@ -663,3 +663,24 @@ func TestPrctlRetInt(t *testing.T) {
 		t.Fatalf("unexpected return from prctl; got %v, expected %v", v, 1)
 	}
 }
+
+func TestOpenat2(t *testing.T) {
+	how := &unix.OpenHow{
+		Flags:   unix.O_RDONLY,
+		Resolve: unix.RESOLVE_NO_MAGICLINKS,
+	}
+	fd, err := unix.Openat2(-1, "/proc/self/fd/1", how)
+	switch err {
+	case nil:
+		unix.Close(fd)
+		t.Fatal("expected ELOOP, got nil")
+	case unix.ELOOP:
+		return
+	case unix.ENOSYS:
+		t.Skip("old kernel? (need Linux >= 5.6)")
+	case unix.ENOENT:
+		t.Skipf("openat2: %v, skipping test", err)
+	default:
+		t.Fatalf("unexpected error from openat2: %v", err)
+	}
+}