macvlan: Set IFF_UNICAST_FLT flag to prevent unnecessary promisc mode.

[ Upstream commit 87ab7f6f2874f1115817e394a7ed2dea1c72549e ]

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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>