mbuf: fix dynamic fields and flags with multiprocess

mbuf: fix dynamic fields and flags with multiprocess

The dynamic flag management is broken if rte_mbuf_dynflag_lookup()
is done in a secondary process because the local pointer to
the memzone is not ever initialized.

Fix it by using the same checks as dynfield_register().
I.e if shared memory zone has not been looked up already,
then discover it.

Fixes: 4958ca3a443a ("mbuf: support dynamic fields and flags")
Cc: stable@dpdk.org
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Olivier Matz <olivier.matz@6wind.com>