xsk: Clear pool even for inactive queues

The number of queues can change by other means, rather than ethtool. For
example, attaching an mqprio qdisc with num_tc > 1 leads to creating
multiple sets of TX queues, which may be then destroyed when mqprio is
deleted. If an AF_XDP socket is created while mqprio is active,
dev->_tx[queue_id].pool will be filled, but then real_num_tx_queues may
decrease with deletion of mqprio, which will mean that the pool won't be
NULLed, and a further increase of the number of TX queues may expose a
dangling pointer.

To avoid any potential misbehavior, this commit clears pool for RX and
TX queues, regardless of real_num_*_queues, still taking into
consideration num_*_queues to avoid overflows.

Fixes: 1c1efc2af158 ("xsk: Create and free buffer pool independently from umem")
Fixes: a41b4f3c58dd ("xsk: simplify xdp_clear_umem_at_qid implementation")
Signed-off-by: Maxim Mikityanskiy <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Acked-by: Björn Töpel <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

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