ring: optimize empty test

ring: optimize empty test

Testing if the ring is empty is as simple as comparing the producer and
consumer pointers.

In theory, this optimization reduces the number of potential cache misses
from 3 to 2 by not having to read r->mask in rte_ring_count().

The modification of this function were also discussed in the RFC here:
https://mails.dpdk.org/archives/dev/2020-April/165752.html

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>