xfrm: Don't queue retransmitted packets if the original is still on the host

It does not make sense to queue retransmitted packets if the
original packet is still in some queue of this host. So add
a check to xdst_queue_output() and drop the packet if the
original packet is not yet sent.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Acked-by: Eric Dumazet <edumazet@google.com>