net: hinic: avoid kernel hung in hinic_get_stats64()

When using hinic device as a bond slave device, and reading device stats
of master bond device, the kernel may hung.

The kernel panic calltrace as follows:
Kernel panic - not syncing: softlockup: hung tasks
Call trace:
  native_queued_spin_lock_slowpath+0x1ec/0x31c
  dev_get_stats+0x60/0xcc
  dev_seq_printf_stats+0x40/0x120
  dev_seq_show+0x1c/0x40
  seq_read_iter+0x3c8/0x4dc
  seq_read+0xe0/0x130
  proc_reg_read+0xa8/0xe0
  vfs_read+0xb0/0x1d4
  ksys_read+0x70/0xfc
  __arm64_sys_read+0x20/0x30
  el0_svc_common+0x88/0x234
  do_el0_svc+0x2c/0x90
  el0_svc+0x1c/0x30
  el0_sync_handler+0xa8/0xb0
  el0_sync+0x148/0x180

And the calltrace of task that actually caused kernel hungs as follows:
  __switch_to+124
  __schedule+548
  schedule+72
  schedule_timeout+348
  __down_common+188
  __down+24
  down+104
  hinic_get_stats64+44 [hinic]
  dev_get_stats+92
  bond_get_stats+172 [bonding]
  dev_get_stats+92
  dev_seq_printf_stats+60
  dev_seq_show+24
  seq_read_iter+964
  seq_read+220
  proc_reg_read+164
  vfs_read+172
  ksys_read+108
  __arm64_sys_read+28
  el0_svc_common+132
  do_el0_svc+40
  el0_svc+24
  el0_sync_handler+164
  el0_sync+324

When getting device stats from bond, kernel will call bond_get_stats().
It first holds the spinlock bond->stats_lock, and then call
hinic_get_stats64() to collect hinic device's stats.
However, hinic_get_stats64() calls `down(&nic_dev->mgmt_lock)` to
protect its critical section, which may schedule current task out.
And if system is under high pressure, the task cannot be woken up
immediately, which eventually triggers kernel hung panic.

Since previous patch has replaced hinic_dev.tx_stats/rx_stats with local
variable in hinic_get_stats64(), there is nothing need to be protected
by lock, so just removing down()/up() is ok.

Fixes: edd384f6 ("net-next/hinic: Add ethtool and stats")
Signed-off-by: Qiao Ma <mqaio@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/huawei/hinic/hinic_main.c

@@ -842,13 +842,9 @@ static void hinic_get_stats64(struct net_device *netdev,
 	struct hinic_rxq_stats nic_rx_stats = {};
 	struct hinic_txq_stats nic_tx_stats = {};
 
-	down(&nic_dev->mgmt_lock);
-
 	if (nic_dev->flags & HINIC_INTF_UP)
 		gather_nic_stats(nic_dev, &nic_rx_stats, &nic_tx_stats);
 
-	up(&nic_dev->mgmt_lock);
-
 	stats->rx_bytes   = nic_rx_stats.bytes;
 	stats->rx_packets = nic_rx_stats.pkts;
 	stats->rx_errors  = nic_rx_stats.errors;