driver: macvlan: Remove duplicated IFF_UP condition check in macvlan_forward_source

The function macvlan_forward_source_one has already checked the flag
IFF_UP, so needn't check it outside in macvlan_forward_source too.

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