zebra: Fix crash with certain types of tunnels

zebra: Fix crash with certain types of tunnels

Zebra did not have a handler for tunnels in v6 for
some reason.  Add code to handle the broadcast address
for both addition and deletion.

This appears to fix the crash.  There might still need
to be some work to make the code `work` properly for
this type of tunnel.

Fixes: #2063
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>