net: app: Add IP header to packets received by TLS/DTLS tunnel

The IP header was stripped by _net_app_ssl_mux() when it received
IP packet. This is fine but if the application expects the get
the IP header, then there is a problem. Fix this by saving IP
header to ssl_context and then putting it back in front of the
packet when the data is passed to application.
Note that this IP header is not used by net_app when the packet
is sent because TLS/DTLS creates a tunnel for transferring packets
and user can only sent packets via this tunnel.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>