Suspend to idle was found to not work on Goldmont CPU recently.

The issue happens due to:

 1. On Goldmont the CPU in idle can only be woken up via IPIs,
    not POLLING mode, due to commit 08e237fa ("x86/cpu: Add
    workaround for MONITOR instruction erratum on Goldmont based
    CPUs")

 2. When the CPU is entering suspend to idle process, the
    _TIF_POLLING_NRFLAG remains on, because cpuidle_enter_s2idle()
    doesn't match call_cpuidle() exactly.

 3. Commit b2a02fc4 ("smp: Optimize send_call_function_single_ipi()")
    makes use of _TIF_POLLING_NRFLAG to avoid sending IPIs to idle
    CPUs.

 4. As a result, some IPIs related functions might not work
    well during suspend to idle on Goldmont. For example, one
    suspected victim:

    tick_unfreeze() -> timekeeping_resume() -> hrtimers_resume()
    -> clock_was_set() -> on_each_cpu() might wait forever,
    because the IPIs will not be sent to the CPUs which are
    sleeping with _TIF_POLLING_NRFLAG set, and Goldmont CPU
    could not be woken up by only setting _TIF_NEED_RESCHED
    on the monitor address.

To avoid that, clear the _TIF_POLLING_NRFLAG flag before invoking
enter_s2idle_proper() in cpuidle_enter_s2idle() in analogy with the
call_cpuidle() code flow.

Fixes: b2a02fc4 ("smp: Optimize send_call_function_single_ipi()")
Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Suggested-by: Rafael J. Wysocki <rafael@kernel.org>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
[ rjw: Subject / changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>