bonding: Turn on IFF_UNICAST_FLT on bond devices

Bonding devices manage the unicast filters of the underlying
interfaces, but do not turn on IFF_UNICAST_FLT flag.  Thus
anytime a unicast address is added to the bond, the bond is
places in promiscuous mode.

Turn on IFF_UNICAST_FLT on the bond device so that the bond does
not go into promiscuous mode needlesly.  If an underlying device
does not support unicast filtering, that device will automaticall
enter promiscuous mode already.

Signed-off-by: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>