net: hinic: avoid kernel hung in hinic_get_stats64()

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

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.