zebra: fix checking of clients subscribed to receive interface info

zebra: fix checking of clients subscribed to receive interface info

client->ifinfo is a VRF bitmap, hence we need to use
vrf_bitmap_check() to check if a client is subscribed to receive
interface information for a particular VRF. Just checking if
the client->ifinfo value is set will always succeed since it's
a pointer initialized by zserv_client_create(). With this fix,
we'll stop sending interface messages from all VRFs to all clients,
even those that didn't subscribe to it.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>