net-sysfs: move the xps cpus/rxqs retrieval in a common function

Most of the xps_cpus_show and xps_rxqs_show functions share the same
logic. Having it in two different functions does not help maintenance.
This patch moves their common logic into a new function, xps_queue_show,
to improve this.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>