x/sys/unix: intermittent failures with EBADF in TestOpenByHandleAt since 2021-11-05

[bcmills]

--- FAIL: TestOpenByHandleAt (0.00s)
    syscall_linux_test.go:705: mountID: 752, handle: size=12, type=1, bytes="U\x81\x80E\x9d3\x00\x00\x00\x00\x00\x00"
    --- FAIL: TestOpenByHandleAt/clone=true (0.00s)
        syscall_linux_test.go:725: opened fd 7
        syscall_linux_test.go:729: read : bad file descriptor
FAIL
FAIL	golang.org/x/sys/unix	0.635s

greplogs --dashboard -md -l -e 'FAIL: TestOpenByHandleAt'

2021-11-05T20:06:27-c75c477-7be227c/linux-amd64-race
2021-11-05T19:01:13-c75c477-93bab8a/linux-amd64-race
2021-11-05T04:20:33-7861aae-0a5ca24/linux-amd64-race

[bcmills]

This looks like another *os.File finalizer race caught out by @mknyszek's GC pacer changes.

https://github.com/golang/sys/blob/39ccf1dd6fa64b525e017b629b9a83409d294c7e/unix/syscall_linux_test.go#L723-L727:

			defer unix.Close(fd)

			t.Logf("opened fd %v", fd)
			f := os.NewFile(uintptr(fd), "")
			slurp, err := ioutil.ReadAll(f)

[gopherbot]

Change https://golang.org/cl/361995 mentions this issue: unix: fix finalizer fd close bug in TestOpenByHandleAt