net: app: Fix for net_app_get_net_buf()

The documentation says that the API will automatically append the
net_buf fragment to the end of network packet fragment chain.
This was not the case and current only user for this API in
echo-server sample appended the fragment itself. The fix is to
automatically append the fragment to the end of fragment chain.

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