port: fix reassembly and fragmentation

port: fix reassembly and fragmentation

Bug fixes for ring ports with IPv4/IPv6 reassembly support.
Previous implementation can't work properly due to incorrect choosing
process function.
Also, assuming that, when processing ip packet, ip header is know we can
set l3_len parameter here.

Fix usage RTE_MBUF_METADATA_* macros due to redefinition the macros.

Fixes: 50f54a84dfb7 ("port: add IPv6 reassembly port")
Fixes: ba92d511ddac ("port: move metadata offset reference at mbuf head")
Signed-off-by: Piotr Azarewicz <piotrx.t.azarewicz@intel.com>
Acked-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>