8021q: fix vlan device to inherit the unicast filtering capability flag

This bug is observed on running FCoE over a VLAN device associated w/
a real device that has IFF_UNICAST_FLT set since FCoE would add unicast
address such as FLOGI MAC to the VLAN interface that FCoE is on. Since
currently, VLAN device is not inheriting the IFF_UNICAST_FLT flag from the
parent real device even though the real device is capable of doing unicast
filtering. This forces the VLAN device and its real device go to promiscuous
mode unnecessarily even the added address is actually being added to the
available unicast filter table in real device.

Signed-off-by: Yi Zou <yi.zou@intel.com>
Cc: devel@open-fcoe.org
Signed-off-by: David S. Miller <davem@davemloft.net>