net/mlx5e: fix BQL reset on SQ re-activation

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5e: fix BQL reset on SQ re-activation

mlx5e_queue_start() deactivates and re-activates all channels but closes
only the queue being restarted. mlx5e_activate_txqsq() then
unconditionally calls netdev_tx_reset_queue(), zeroing the BQL counters
of channels that kept their in-flight TX WQEs. The next completion then
over-charges and trips the BUG_ON() in dql_completed():

kernel BUG at lib/dynamic_queue_limits.c:99!
RIP: 0010:dql_completed+0x23d/0x280
Call Trace:
<IRQ>
mlx5e_poll_tx_cq+0x668/0xa60
mlx5e_napi_poll+0x5b/0x7b0
net_rx_action+0x15a/0x580

Reset BQL only when the SQ has no bytes in flight (sq->cc == sq->pc).

In the case that reset is skipped, the outstanding WQEs will eventually
complete and rebalance the dql. The dql->limit is carried across the
reset.