The logic from switchdev_handle_port_obj_add_foreign() is directly
adapted from switchdev_handle_fdb_event_to_device(), which already
detects events on foreign interfaces and reoffloads them towards the
switchdev neighbors.

However, when we have a simple br0 <-> bond0 <-> swp0 topology and the
switchdev_handle_port_obj_add_foreign() gets called on bond0, we get
stuck into an infinite recursion:

1. bond0 does not pass check_cb(), so we attempt to find switchdev
   neighbor interfaces. For that, we recursively call
   __switchdev_handle_port_obj_add() for bond0's bridge, br0.

2. __switchdev_handle_port_obj_add() recurses through br0's lowers,
   essentially calling __switchdev_handle_port_obj_add() for bond0

3. Go to step 1.

This happens because switchdev_handle_fdb_event_to_device() and
switchdev_handle_port_obj_add_foreign() are not exactly the same.
The FDB event helper special-cases LAG interfaces with its lag_mod_cb(),
so this is why we don't end up in an infinite loop - because it doesn't
attempt to treat LAG interfaces as potentially foreign bridge ports.

The problem is solved by looking ahead through the bridge's lowers to
see whether there is any switchdev interface that is foreign to the @dev
we are currently processing. This stops the recursion described above at
step 1: __switchdev_handle_port_obj_add(bond0) will not create another
call to __switchdev_handle_port_obj_add(br0). Going one step upper
should only happen when we're starting from a bridge port that has been
determined to be "foreign" to the switchdev driver that passes the
foreign_dev_check_cb().

Fixes: c4076cdd ("net: switchdev: introduce switchdev_handle_port_obj_{add,del} for foreign interfaces")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>