packet: restore packet statistics tp_packets to include drops

getsockopt PACKET_STATISTICS returns tp_packets + tp_drops. Commit
ee80fbf3 ("packet: account statistics only in tpacket_stats_u")
cleaned up the getsockopt PACKET_STATISTICS code.
This also changed semantics. Historically, tp_packets included
tp_drops on return. The commit removed the line that adds tp_drops
into tp_packets.

This patch reinstates the old semantics.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>