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/ca728d81-80e8-3767-d5e-d44f6ad96e43@ssi.bg/Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>