ipvlan: Add new func ipvlan_is_valid_dev instead of duplicated codes

There are multiple duplicated condition checks in the current codes, so
I add the new func ipvlan_is_valid_dev instead of the duplicated codes to
check if the netdev is real ipvlan dev.

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