ring: add optional yield to avoid spin forever

ring: add optional yield to avoid spin forever

Add a sched_yield() syscall if the thread spins for too long,
waiting other thread to finish its operations on the ring.
That gives pre-empted thread a chance to proceed and finish
with ring enqueue/dequeue operation.
The purpose is to reduce contention on the ring.
By ring_perf_test, it doesn't shows additional perf penalty.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>