net: switchdev: fix FDB entries towards foreign ports not getting propagated to us

The newly introduced switchdev_handle_fdb_{add,del}_to_device helpers
solved a problem but introduced another one. They have a severe design
bug: they do not propagate FDB events on foreign interfaces to us, i.e.
this use case:

         br0
        /   \
       /     \
      /       \
     /         \
   swp0       eno0
(switchdev)  (foreign)

when an address is learned on eno0, what is supposed to happen is that
this event should also be propagated towards swp0. Somehow I managed to
convince myself that this did work correctly, but obviously it does not.

The trouble with foreign interfaces is that we must reach a switchdev
net_device pointer through a foreign net_device that has no direct
upper/lower relationship with it. So we need to do exploratory searching
through the lower interfaces of the foreign net_device's bridge upper
(to reach swp0 from eno0, we must check its upper, br0, for lower
interfaces that pass the check_cb and foreign_dev_check_cb). This is
something that the previous code did not do, it just assumed that "dev"
will become a switchdev interface at some point, somehow, probably by
magic.

With this patch, assisted address learning on the CPU port works again
in DSA:

ip link add br0 type bridge
ip link set swp0 master br0
ip link set eno0 master br0
ip link set br0 up

[   46.708929] mscc_felix 0000:00:00.5 swp0: Adding FDB entry towards eno0, addr 00:04:9f:05:f4:ab vid 0 as host address

Fixes: 8ca07176 ("net: switchdev: introduce a fanout helper for SWITCHDEV_FDB_{ADD,DEL}_TO_DEVICE")
Reported-by: Eric Woudstra <ericwouds@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>