net: ip: net_pkt: set pkt context, iface and family in net_pkt_get()

Commit 753daa65 ("net: pkt: Compute TX payload data length")
removed the default packet setup on incoming packets when they
belong to the rx_pkt pool.

Let's restore this behavior, as MBEDTLS processing in net_app library
needs to use packet family to determine IP header length on
incoming packets.

NOTE: A future cleanup patch could set the IP header length based
on the context IP family.  However, there are many places in the code
where this is being set, so care should be taken.

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