l2-flood: fix restore vnet buffer's flags in the replication routine

When BUM packets are flooded in the l2 domain, some data should be
kept and restored for recycling in the replication routine.
If l2 bridge domain has multiple interfaces mixed with normal and
vlan tagged, the vlan tag value of the vnet buffer can be changed
while flooding the replicated packets. The change is made to store
and restore the original vlan tag in the replication logic.

Change-Id: I399cf54cd2e74cb44a2be42241bdc4fba85032c5
Signed-off-by: Steve Shin <jonshin@cisco.com>