posix-cpu-timers: Prevent freeing a timer which is queued on the expiry list

Kijo analyzed another race in the POSIX CPU timer code:

Commit bf635681c906 converted cpu_timer::firing from a tristate value to a
boolean. This lost the distinction between "not owned by the firing list"
and "still owned, but delivery was canceled". The resulting race is:

    expiry handler              timer_settime()        timer_delete()
    --------------              ---------------        --------------
    collect timer onto
    private firing list
    firing = true
                                observes firing = true
                                firing = false
                                return TIMER_RETRY
                                wait for handler
                                                       observes firing = false
                                                       finish deletion
                                                       unhash and free timer
    resume list traversal
    read freed elist.next
    -> UAF

The firing bit is clearly the wrong indicator since that commit.

Check whether the timer is queued on the expiry list or not instead. If it
is queued clear the firing bit to prevent signal delivery as before and
return TIMER_RETRY so the caller unlocks the timer which allows the expiry
code to make progress and remove it from the list.

Fixes: bf635681c906 ("posix-cpu-timers: Cleanup the firing logic")
Reported-by: Kijo Park <red993688@gmail.com>
Debugged-by: Kijo Park <red993688@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Tested-by: Kijo Park <red993688@gmail.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Cc: stable@vger.kernel.org