netdevsim: implement peer queue flow control

Add flow control mechanism between paired netdevsim devices to stop the
TX queue during high traffic scenarios. When a receive queue becomes
congested (approaching NSIM_RING_SIZE limit), the corresponding transmit
queue on the peer device is stopped using netif_subqueue_try_stop().

Once the receive queue has sufficient capacity again, the peer's
transmit queue is resumed with netif_tx_wake_queue().

Key changes:
  * Add nsim_stop_peer_tx_queue() to pause peer TX when RX queue is full
  * Add nsim_start_peer_tx_queue() to resume peer TX when RX queue drains
  * Implement queue mapping validation to ensure TX/RX queue count match
  * Wake all queues during device unlinking to prevent stuck queues
  * Use RCU protection when accessing peer device references
  * wake the queues when changing the queue numbers
  * Remove IFF_NO_QUEUE given it will enqueue packets now

The flow control only activates when devices have matching TX/RX queue
counts to ensure proper queue mapping.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250711-netdev_flow_control-v3-1-aa1d5a155762@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>