[release-branch.go1.12] os: don't return ENOENT if directory removed before Fstatat

Updates #30197

Change-Id: I08b592fbd477d6879eb5d3b7fcbbc8322ea90103
Reviewed-on: https://go-review.googlesource.com/c/162078
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
(cherry picked from commit cf4dc255)
Reviewed-on: https://go-review.googlesource.com/c/162197

src/os/removeall_at.go

@@ -71,6 +71,9 @@ func removeAllFrom(parent *File, path string) error {
 	var statInfo syscall.Stat_t
 	statErr := unix.Fstatat(parentFd, path, &statInfo, unix.AT_SYMLINK_NOFOLLOW)
 	if statErr != nil {
+		if IsNotExist(statErr) {
+			return nil
+		}
 		return statErr
 	}
 	if statInfo.Mode&syscall.S_IFMT != syscall.S_IFDIR {