packet: less dev_put() calls

- packet_sendmsg_spkt() can use dev_get_by_name_rcu() to avoid touching device refcount.

- packet_getname_spkt() & packet_getname() can use dev_get_by_index_rcu() to
  avoid touching device refcount too.

tpacket_snd() & packet_snd() can not use RCU yet because they can sleep when
allocating skb.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>