ipv4: remove the routes on secondary promotion

The secondary address promotion relies on fib_sync_down_addr
to remove all routes created for the secondary addresses when
the old primary address is deleted. It does not happen for cases
when the primary address is also in another subnet. Fix that
by deleting local and broadcast routes for all secondaries while
they are on device list and by faking that all addresses from
this subnet are to be deleted. It relies on fib_del_ifaddr being
able to ignore the IPs from the concerned subnet while checking
for duplication.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>