net: pkt_filter: Add statistics support to packet filter

As the network packet filter drops packets without any indication
that the packet is dropped, it can be difficult to monitor what
is going on in the system when receiving data. The user can
now monitor the statistics and see if packets are being dropped
because of packet filter activity.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>