packet: make packet too small warning match condition

The expression in ll_header_truncated() tests less than or equal, but
the warning prints less than. Update the warning.

Reported-by: Jouni Malinen <jkmalinen@gmail.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>