zebra: L3VNI to L2VNI conversion is not handled

zebra: L3VNI to L2VNI conversion is not handled

After removal of L3VNI config, the VNI should become an L2VNI if a VxLAN
interface is present for the VNI. This case is not handled in the code.

Changes:
1. After unconfiguring L3VNI, create an L2VNI if VxLAN interface is present
for the VNI.
2. Trigger an update to BGP.
3. Read MAC and ARP entries from kernel.

This PR fixes the issue only for route type-2, 3 and 5. This PR does not address
states regarding route type-1, 4 and multicast group for VxLAN interface.

Signed-off-by: Ameya Dharkar <adharkar@vmware.com>