locking/ww-mutex: Limit stress test to 2 seconds

Use a timeout rather than a fixed number of loops to avoid running for
very long periods, such as under the kbuilder VMs.

Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170310105733.6444-1-chris@chris-wilson.co.uk


Signed-off-by: Ingo Molnar <mingo@kernel.org>