net: switchdev: introduce helper for checking dynamically learned FDB entries

It is a bit difficult to understand what DSA checks when it tries to
avoid installing dynamically learned addresses on foreign interfaces as
local host addresses, so create a generic switchdev helper that can be
reused and is generally more readable.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>