timerfd: Handle relative timers with CONFIG_TIME_LOW_RES proper

commit b62526ed upstream.

Helge reported that a relative timer can return a remaining time larger than
the programmed relative time on parisc and other architectures which have
CONFIG_TIME_LOW_RES set. This happens because we add a jiffie to the resulting
expiry time to prevent short timeouts.

Use the new function hrtimer_expires_remaining_adjusted() to calculate the
remaining time. It takes that extra added time into account for relative
timers.
Reported-and-tested-by: Helge Deller <deller@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: John Stultz <john.stultz@linaro.org>
Cc: linux-m68k@lists.linux-m68k.org
Cc: dhowells@redhat.com
Link: http://lkml.kernel.org/r/20160114164159.354500742@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/timerfd.c

@@ -123,7 +123,7 @@ static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
 {
 	ktime_t remaining;
 
-	remaining = hrtimer_expires_remaining(&ctx->tmr);
+	remaining = hrtimer_expires_remaining_adjusted(&ctx->tmr);
 	return remaining.tv64 < 0 ? ktime_set(0, 0): remaining;
 }