ring: add a function to return the ring size

ring: add a function to return the ring size

Applications and other libraries should not be reading inside the
rte_ring structure directly to get the ring size. Instead add a fn
to allow it to be queried.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>