driver: ipvlan: Define common functions to decrease duplicated codes used to add or del IP address

There are some duplicated codes in ipvlan_add_addr6/4 and
ipvlan_del_addr6/4. Now define two common functions ipvlan_add_addr
and ipvlan_del_addr to decrease the duplicated codes.
It could be helful to maintain the codes.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>