macvlan: Set IFF_UNICAST_FLT flag to prevent unnecessary promisc mode.

Macvlan already supports hw address filters.  Set the IFF_UNICAST_FLT
so that it doesn't needlesly enter PROMISC mode when macvlans are
stacked.

Signed-of-by: Vlad Yasevich <vyasevic@redhat.com>

Signed-off-by: David S. Miller <davem@davemloft.net>