mbuf: rename RTE_MBUF_SCATTER_GATHER into RTE_MBUF_REFCNT

mbuf: rename RTE_MBUF_SCATTER_GATHER into RTE_MBUF_REFCNT

It seems that RTE_MBUF_SCATTER_GATHER is not the proper name for the
feature it provides. "Scatter gather" means that data is stored using
several buffers. RTE_MBUF_REFCNT seems to be a better name for that
feature as it provides a reference counter for mbufs.

The macro RTE_MBUF_SCATTER_GATHER is poisoned to ensure this
modification is seen by drivers or applications using it.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>