hrtimer: check relative timeouts for overflow

Various user space callers ask for relative timeouts. While we fixed
that overflow issue in hrtimer_start(), the sites which convert
relative user space values to absolute timeouts themself were uncovered.

Instead of putting overflow checks into each place add a function
which does the sanity checking and convert all affected callers to use
it.

Thanks to Frans Pop, who reported the problem and tested the fixes.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ingo Molnar <mingo@elte.hu>
Tested-by: Frans Pop <elendil@planet.nl>

kernel/futex.c

@@ -2116,7 +2116,7 @@ asmlinkage long sys_futex(u32 __user *uaddr, int op, u32 val,
 
 		t = timespec_to_ktime(ts);
 		if (cmd == FUTEX_WAIT)
-			t = ktime_add(ktime_get(), t);
+			t = ktime_add_safe(ktime_get(), t);
 		tp = &t;
 	}
 	/*