ipv4: Handle route deletion notification during flush

In a similar fashion to previous patch, when a route is deleted as part
of table flushing, promote the next route in the list, if exists.
Otherwise, simply emit a delete notification.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>