Changes:
  - Save CPU cycles by computing the return value while scanning the
    input iovec, rather than calling iov_size() at the end.
  - Remove check for s->tx != NULL, because it cannot happen.
  - Cache ring->tail in a local variable and use it to check for
    space in the TX ring. The use of nm_ring_empty() was invalid,
    because nobody is updating ring->cur and ring->head at that point.
  - In case we run out of netmap slots in the middle of a packet,
    move the wake-up point by advancing ring->cur, but do not
    expose the incomplete packet (i.e., by updating also ring->head).

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