hrtimers: Special-case zero length sleeps

hrtimers: Special-case zero length sleeps

sleep(0) is a common construct used by applications that want to trigger
the scheduler.  sched_yield() might make more sense, but only appeared in
POSIX.1-2001 and so plenty of example code still uses the sleep(0) form.

This wouldn't normally be a problem, but it means that event-driven
applications that are merely trying to avoid starving other processes may
actually end up sleeping due to having large timer_slack values.  Special-
casing this seems reasonable.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>