alarmtimer: Prevent overflow of relative timers

Andrey reported a alartimer related RCU stall while fuzzing the kernel with
syzkaller.

The reason for this is an overflow in ktime_add() which brings the
resulting time into negative space and causes immediate expiry of the
timer. The following rearm with a small interval does not bring the timer
back into positive space due to the same issue.

This results in a permanent firing alarmtimer which hogs the CPU.

Use ktime_add_safe() instead which detects the overflow and clamps the
result to KTIME_SEC_MAX.

Reported-by: Andrey Konovalov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Kostya Serebryany <[email protected]>
Cc: syzkaller <[email protected]>
Cc: John Stultz <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/[email protected]

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