zebra: Format netlink requests correctly

zebra: Format netlink requests correctly

When zebra issues read (GET) requests to the kernel using the netlink
interface, it is incorrect to format all of them in a generic manner
using 'struct ifinfomsg' or 'struct rtgenmsg'. Rather, messages for a
particular entity (e.g., routes) should use the corresponding structure
for encoding (e.g., 'struct rtmsg'). Of course, this has to correlate
with what the kernel expects.

In the absence of this, there is the possibility of sending extraneous
information in the request which the kernel wouldn't like.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Reviewed-by:   David Ahern <dsa@cumulusnetworks.com>