virtio-net: avoid unnecessary sg initialzation
virtio-net: avoid unnecessary sg initialzation

Usually an skb does not have up to MAX_SKB_FRAGS frags. So no need to
initialize the unuse part of sg. This patch initialize the sg based on
the real number it will used:

- during xmit, it could be inferred from nr_frags and can_push.
- for small receive buffer, it will also be 2.

Cc: Michael S. Tsirkin <255103e50249e3d658441816e0597170ebfc16ef@redhat.com>
Signed-off-by: Jason Wang <808efd45667dbd241e4e026b5673172419ecb43e@redhat.com>
Signed-off-by: David S. Miller <fe08d3c717adf2ae63592e4c9aec6e3e404d8e3e@davemloft.net>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs