net: use DEBUG_NET_WARN_ON_ONCE() in dev_loopback_xmit()

One check in dev_loopback_xmit() has not caught issues
in the past.

Keep it for CONFIG_DEBUG_NET=y builds only.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>