mbuf: promote some helpers to stable

mbuf: promote some helpers to stable

Those accessors have been introduced more than two years ago
(rte_mbuf_to_priv in v18.08, rte_mbuf_*_addr* in v19.02).
Time to mark them stable.

rte_mbuf_to_baddr() could be removed, but since we lack a deprecation
notice, keep it as a simple wrapper.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>