[PACKET]: Fix /proc/net/packet crash due to bogus private pointer

The seq_open_net patch changed the meaning of seq->private.
Unfortunately it missed two spots in AF_PACKET, which still
used the old way of dereferencing seq->private, thus causing
weird and wonderful crashes when reading /proc/net/packet.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>