macvlan: remove redundant null check on data

Because macvlan_common_newlink() and macvlan_changelink() already
checked NULL data parameter, so the additional check is unnecessary,
just remove it.

Fixes: 79cf79ab

 ("macvlan: add source mode")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>