ethtool: rss: report info about additional contexts from XArray

IOCTL already uses the XArray when reporting info about additional
contexts. Do the same thing in netlink code.

Reviewed-by: Edward Cree <ecree.xilinx@gmail.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>