[dpdk-stable] missing ring memory barrier in 17.11

[Thomas Monjalon]

Hi Luca, Gavin,

It seems the fix 85cffb2eccd9 ("ring: enforce reading tail before slots")
should be backported in 17.11.

This patch is missing because the root cause is wrongly identified.
It is referenced as
	Fixes: c9fb3c62896f ("ring: move code in a new header file")
in 18.02, but in reality it is from 17.05:
	Fixes: 0dfc98c507b1 ("ring: separate out head index manipulation")

Do you agree?