ipv4: optimize route adding on secondary promotion

Optimize the calling of fib_add_ifaddr for all
secondary addresses after the promoted one to start from
their place, not from the new place of the promoted
secondary. It will save some CPU cycles because we
are sure the promoted secondary was first for the subnet
and all next secondaries do not change their place.

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