quagga: Check for existence of IFLA_INFO_SLAVE_KIND

quagga: Check for existence of IFLA_INFO_SLAVE_KIND

IFLA_INFO_SLAVE_KIND is a new type of netlink message
If the kernel makes it available compile in support
else we'll just silently do the right thing.

Additionally reduce the test cases for netlink by 1
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>
Reviewed-by: David Ahern <dsa@cumulusnetworks.com>
Reviewed-by: Don Slice <dslice@cumulusnetworks.com>
Reviewed-by: Vivek Venkatraman <vivek@cumulusnetworks.com>