netfilter: rpfilter: mask ecn bits before fib lookup

RT_TOS() only masks one of the two ECN bits. Therefore rpfilter_mt()
treats Not-ECT or ECT(1) packets in a different way than those with
ECT(0) or CE.

Reproducer:

  Create two netns, connected with a veth:
  $ ip netns add ns0
  $ ip netns add ns1
  $ ip link add name veth01 netns ns0 type veth peer name veth10 netns ns1
  $ ip -netns ns0 link set dev veth01 up
  $ ip -netns ns1 link set dev veth10 up
  $ ip -netns ns0 address add 192.0.2.10/32 dev veth01
  $ ip -netns ns1 address add 192.0.2.11/32 dev veth10

  Add a route to ns1 in ns0:
  $ ip -netns ns0 route add 192.0.2.11/32 dev veth01

  In ns1, only packets with TOS 4 can be routed to ns0:
  $ ip -netns ns1 route add 192.0.2.10/32 tos 4 dev veth10

  Ping from ns0 to ns1 works regardless of the ECN bits, as long as TOS
  is 4:
  $ ip netns exec ns0 ping -Q 4 192.0.2.11   # TOS 4, Not-ECT
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 5 192.0.2.11   # TOS 4, ECT(1)
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 6 192.0.2.11   # TOS 4, ECT(0)
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 7 192.0.2.11   # TOS 4, CE
    ... 0% packet loss ...

  Now use iptable's rpfilter module in ns1:
  $ ip netns exec ns1 iptables-legacy -t raw -A PREROUTING -m rpfilter --invert -j DROP

  Not-ECT and ECT(1) packets still pass:
  $ ip netns exec ns0 ping -Q 4 192.0.2.11   # TOS 4, Not-ECT
    ... 0% packet loss ...
  $ ip netns exec ns0 ping -Q 5 192.0.2.11   # TOS 4, ECT(1)
    ... 0% packet loss ...

  But ECT(0) and ECN packets are dropped:
  $ ip netns exec ns0 ping -Q 6 192.0.2.11   # TOS 4, ECT(0)
    ... 100% packet loss ...
  $ ip netns exec ns0 ping -Q 7 192.0.2.11   # TOS 4, CE
    ... 100% packet loss ...

After this patch, rpfilter doesn't drop ECT(0) and CE packets anymore.

Fixes: 8f97339d ("netfilter: add ipv4 reverse path filter match")
Signed-off-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>