zebra: Don't kill the global rtadv socket when a vrf is deleted

zebra: Don't kill the global rtadv socket when a vrf is deleted

The rtadv code has two types of sockets:
a) namespace -> Where each zvrf get's it's own socket
b) vrf lite -> Where we get 1 socket for everything

When we were terminating a vrf we were *always*
killing the (b) socket.  This is a mistake in
that other vrf's may need to be communicating.

Modify the code on vrf shutdown to only disable
that vrf's event processing and when we actually
terminate we shut the socket.

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