ice: xsk: Fix indexing in ice_tx_xsk_pool()

Ice driver tries to always create XDP rings array to be
num_possible_cpus() sized, regardless of user's queue count setting that
can be changed via ethtool -L for example.

Currently, ice_tx_xsk_pool() calculates the qid by decrementing the
ring->q_index by the count of XDP queues, but ring->q_index is set to 'i
+ vsi->alloc_txq'.

When user did ethtool -L $IFACE combined 1, alloc_txq is 1, but
vsi->num_xdp_txq is still num_possible_cpus(). Then, ice_tx_xsk_pool()
will do OOB access and in the final result ring would not get xsk_pool
pointer assigned. Then, each ice_xsk_wakeup() call will fail with error
and it will not be possible to get into NAPI and do the processing from
driver side.

Fix this by decrementing vsi->alloc_txq instead of vsi->num_xdp_txq from
ring-q_index in ice_tx_xsk_pool() so the calculation is reflected to the
setting of ring->q_index.

Fixes: 22bf877e

 ("ice: introduce XDP_TX fallback path")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220328142123.170157-5-maciej.fijalkowski@intel.com