vxlan: don't bypass encapsulation for multi- and broadcasts

The multicast and broadcast packets may have RTCF_LOCAL set in rt_flags
and therefore will be sent out bypassing encapsulation. This breaks
delivery of packets sent to the vxlan multicast group.
Disabling encapsulation bypass for multicasts and broadcasts fixes the
issue.

Signed-off-by: Mike Rapoport <mike.rapoport@ravellosystems.com>
Tested-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: Sridhar Samudrala <sri@us.ibm.com>
Tested-by: Sridhar Samudrala <sri@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>