net: ip: Handle delivery_time in ip defrag

A latter patch will postpone the delivery_time clearing until the stack
knows the skb is being delivered locally. That will allow other kernel
forwarding path (e.g. ip[6]_forward) to keep the delivery_time also.

An earlier attempt was to do skb_clear_delivery_time() in
ip_local_deliver() and ip6_input(). The discussion [0] requested
to move it one step later into ip_local_deliver_finish()
and ip6_input_finish() so that the delivery_time can be kept
for the ip_vs forwarding path also.

To do that, this patch also needs to take care of the (rcv) timestamp
usecase in ip_is_fragment(). It needs to expect delivery_time in
the skb->tstamp, so it needs to save the mono_delivery_time bit in
inet_frag_queue such that the delivery_time (if any) can be restored
in the final defragmented skb.

[Note that it will only happen when the locally generated skb is looping
from egress to ingress over a virtual interface (e.g. veth, loopback...),
skb->tstamp may have the delivery time before it is known that it will
be delivered locally and received by another sk.]

[0]: https://lore.kernel.org/netdev/[e-mail]/

Signed-off-by: Martin KaFai Lau <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/inet_frag.h

···
  * @stamp: timestamp of the last received fragment
  * @len: total length of the original datagram
  * @meat: length of received fragments so far
+ * @mono_delivery_time: stamp has a mono delivery time (EDT)
  * @flags: fragment queue flags
  * @max_size: maximum received fragment size
  * @fqdir: pointer to struct fqdir
···
 	ktime_t			stamp;
 	int			len;
 	int			meat;
+	u8			mono_delivery_time;
 	__u8			flags;
 	u16			max_size;
 	struct fqdir		*fqdir;

net/ipv4/inet_fragment.c

···
 	skb_mark_not_on_list(head);
 	head->prev = NULL;
 	head->tstamp = q->stamp;
+	head->mono_delivery_time = q->mono_delivery_time;
 }
 EXPORT_SYMBOL(inet_frag_reasm_finish);
 

net/ipv4/ip_fragment.c

···
 		qp->iif = dev->ifindex;
 
 	qp->q.stamp = skb->tstamp;
+	qp->q.mono_delivery_time = skb->mono_delivery_time;
 	qp->q.meat += skb->len;
 	qp->ecn |= ecn;
 	add_frag_mem_limit(qp->q.fqdir, skb->truesize);