net: net_app: set packet data_len on incoming packets

This fixes a bug when calling net_pkt_append[_all] which uses
pkt->data_len as part of the maximum packet length calculation
when the net_context is set.

Without this change the maximum packet length is calculated as 0
(the value of pkt->data_len) and an ENOMEM error is returned.

Signed-off-by: Michael Scott <michael@opensourcefoundries.com>