net: update real_num_rx_queues even when !CONFIG_SYSFS

We used to depend on real_num_rx_queues as a upper bound for sanity
checks.  For AF_XDP socket validation it's useful if the check behaves
the same regardless of CONFIG_SYSFS setting.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>