MINOR: counters: add common struct and flags to {fe,be}_counters_shared

MINOR: counters: add common struct and flags to {fe,be}_counters_shared

fe_counters_shared and be_counters_shared may share some common members
since they are quite similar, so we add a common struct part shared
between the two. struct counters_shared is added for convenience as
a generic pointer to manipulate common members from fe or be shared
counters pointer.

Also, the first common member is added: shared fe and be counters now
have a flags member.