net/nfp: fix releasing muti-segment mbufs

net/nfp: fix releasing muti-segment mbufs

If segments are used, just mbufs previously linked to head descriptor
of a mbuf chain are released. Other Tx descriptor are used for the
mbuf chain but they keep their linked mbufs without releasing them.

It is not a fatal issue because sooner or later those descriptors will
be head descriptors or just used for a single mbuf packet, then those
linked mbufs will be released.

However, this leads to apps needing bigger mbufs pools and some
confusion about memory requirements. Indeed, because larger pools, some
performance impact could also be expected due to cache misses.

With this patch all Tx descriptors will release linked mbufs inside the
xmit function, and rte_pktmbuf_seg_free is used instead of
rte_pktmbuf_free.

Fixes: 142854c62134 ("nfp: fix freeing multi-mbuf packets")
Cc: stable@dpdk.org
Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>