netdev-dummy: Count rx packets regardless of source

netdev-dummy: Count rx packets regardless of source

This alters the way rx packets are accounted for by
counting them when they are processed by netdev_dummy_rx_recv(),
which seems to be a common path used by all received packets.

Previously accounting was done earlier, in netdev_dummy_receive(),
however this does not appear to count packets that are received via
a socket.

This resolves packet counting errors reported by the following
OFtest tests:

	port_stats.MultiFlowStats
	port_stats.SingleFlowStats
	pktact.WildcardPriorityWithDelete
	pktact.WildcardPriority

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>