zebra: Prevent protodown_rc from going Bzonkas

zebra: Prevent protodown_rc from going Bzonkas

The code that handles the protodown_rc setting for
VRRP interfaces in zebra is sending a interface
to be set into a protodown state *before* the
interface has been learned by the kernel.  Resulting
in crashes when the data plane sends the ctx back
to us saying hey man you are uncool.

Additionally change the protodown code to refuse
to send any protodown_rc codes *until* the interface
has actually been learned about from the kernel.

Ticket: 3582375
Signed-off-by: Donald Sharp <sharpd@nvidia.com>