net: netmap: small improvements netmap_send()

net: netmap: small improvements netmap_send()

This change improves the handling of incomplete multi-slot packets
(e.g. with the NS_MOREFRAG set), by advancing ring->head only on
complete packets. The ring->cur pointer is advanced in any case in
order to acknowledge the kernel and move the wake-up point (thus
avoiding repeated wake-ups).
Also don't be verbose when incomplete packets are found.

Signed-off-by: Vincenzo Maffione <v.maffione@gmail.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>