ena: hide sysctl nodes for unused ENA queues

ena: hide sysctl nodes for unused ENA queues

IO queue related attributes are registered statically at driver attach
with the rest of the ENA specific sysctl nodes. However, the number of
queues can be changed at runtime via the `ena_sysctl_io_queues_nb`
request, leading to a potential exposure of attributes for non-existing
queues.

Introduce a new `ena_sysctl_update_queue_node_nb` function, which
updates the sysctl nodes after the number of queues is altered.
This happens by either registering or unregistering node specific oids,
based on a delta between the previous and current queue count.

NOTE: All unregistered oids must be registered again before the driver
detach, e.g. by another call to this function.

Submitted by: Artur Rojek <ar@semihalf.com>
Obtained from: Semihalf
MFC after: 2 weeks
Sponsored by: Amazon, Inc.