sch_cake: Take advantage of skb->hash where appropriate

While the other fq-based qdiscs take advantage of skb->hash and doesn't
recompute it if it is already set, sch_cake does not.

This was a deliberate choice because sch_cake hashes various parts of the
packet header to support its advanced flow isolation modes. However,
foregoing the use of skb->hash entirely loses a few important benefits:

- When skb->hash is set by hardware, a few CPU cycles can be saved by not
  hashing again in software.

- Tunnel encapsulations will generally preserve the value of skb->hash from
  before the encapsulation, which allows flow-based qdiscs to distinguish
  between flows even though the outer packet header no longer has flow
  information.

It turns out that we can preserve these desirable properties in many cases,
while still supporting the advanced flow isolation properties of sch_cake.
This patch does so by reusing the skb->hash value as the flow_hash part of
the hashing procedure in cake_hash() only in the following conditions:

- If the skb->hash is marked as covering the flow headers (skb->l4_hash is
  set)

AND

- NAT header rewriting is either disabled, or did not change any values
  used for hashing. The latter is important to match local-origin packets
  such as those of a tunnel endpoint.

The immediate motivation for fixing this was the recent patch to WireGuard
to preserve the skb->hash on encapsulation. As such, this is also what I
tested against; with this patch, added latency under load for competing
flows drops from ~8 ms to sub-1ms on an RRUL test over a WireGuard tunnel
going through a virtual link shaped to 1Gbps using sch_cake. This matches
the results we saw with a similar setup using sch_fq_codel when testing the
WireGuard patch.

Fixes: 046f6fd5

 ("sched: Add Common Applications Kept Enhanced (cake) qdisc")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>