samples: net: Fix echo_server reply packet preparation

If the incoming packet fragments doesn't have any link layer
header then build_reply_pkt() will set reply packet fragment's link
layer header space to zero. Which is causing issue in case of IPv4.
ARP layer is trying to fill Ethernet headers, but fragments
ll header length is set to zero.

Just use net_pkt_copy_all() to copy payload. That should be enough.

Fixes #6564

Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>