bonding: drop traffic on backup interface for active-backup mode

For active-backup mode, we transmit on one and only one interface. However,
we might still receive traffic on the backup interface. We should drop them
and strictly process incoming traffic on only the active interface.

Type: fix

Signed-off-by: Steven Luong <sluong@cisco.com>
Change-Id: Idb6b798b30033e84044b151c616be3c157329731