net: add debugging checks in skb_attempt_defer_free()

Make sure skbs that are stored in softnet_data.defer_list
do not have a dst attached.

Also make sure the the skb was orphaned.

Link: https://lore.kernel.org/netdev/CANn89iJuEVe72bPmEftyEJHLzzN=QNR2yueFjTxYXCEpS5S8HQ@mail.gmail.com/T/


Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>