ipvlan: Eliminate duplicated codes with existing function

The recv flow of ipvlan l2 mode performs as same as l3 mode for
non-multicast packet, so use the existing func ipvlan_handle_mode_l3
instead of these duplicated statements in non-multicast case.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: David S. Miller <davem@davemloft.net>