mbuf: optimize refcnt update

mbuf: optimize refcnt update

In __rte_pktmbuf_prefree_seg(), there was an optimization to avoid using
a costly atomic operation when updating the mbuf reference counter if
its value is 1. Indeed, it means that we are the only owner of the mbuf,
and therefore nobody can change it at the same time.

We can generalize this optimization directly in rte_mbuf_refcnt_update()
so the other callers of this function, like rte_pktmbuf_attach(), can
also take advantage of this optimization.

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