dm thin: Fix UAF in run_timer_softirq()

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

dm thin: Fix UAF in run_timer_softirq()

When dm_resume() and dm_destroy() are concurrent, it will
lead to UAF, as follows:

BUG: KASAN: use-after-free in __run_timers+0x173/0x710
Write of size 8 at addr ffff88816d9490f0 by task swapper/0/0
<snip>
Call Trace:
<IRQ>
dump_stack_lvl+0x73/0x9f
print_report.cold+0x132/0xaa2
_raw_spin_lock_irqsave+0xcd/0x160
__run_timers+0x173/0x710
kasan_report+0xad/0x110
__run_timers+0x173/0x710
__asan_store8+0x9c/0x140
__run_timers+0x173/0x710
call_timer_fn+0x310/0x310
pvclock_clocksource_read+0xfa/0x250
kvm_clock_read+0x2c/0x70
kvm_clock_get_cycles+0xd/0x20
ktime_get+0x5c/0x110
lapic_next_event+0x38/0x50
clockevents_program_event+0xf1/0x1e0
run_timer_softirq+0x49/0x90
__do_softirq+0x16e/0x62c
__irq_exit_rcu+0x1fa/0x270
irq_exit_rcu+0x12/0x20
sysvec_apic_timer_interrupt+0x8e/0xc0

One of the concurrency UAF can be shown as below:

use free
do_resume |
__find_device_hash_cell |
dm_get |
atomic_inc(&md->holders) |
| dm_destroy
| __dm_destroy
| if (!dm_suspended_md(md))
| atomic_read(&md->holders)
| msleep(1)
dm_resume |
__dm_resume |
dm_table_resume_targets |
pool_resume |
do_waker #add delay work |
dm_put |
atomic_dec(&md->holders) |
| dm_table_destroy
| pool_dtr
| __pool_dec
| __pool_destroy
| destroy_workqueue
| kfree(pool) # free pool
time out
__do_softirq
run_timer_softirq # pool has already been freed

This can be easily reproduced using:
1. create thin-pool
2. dmsetup suspend pool
3. dmsetup resume pool
4. dmsetup remove_all # Concurrent with 3

The root cause of this UAF bug is that dm_resume() adds timer after
dm_destroy() skips cancelling the timer because of suspend status.
After timeout, it will call run_timer_softirq(), however pool has
already been freed. The concurrency UAF bug will happen.

Therefore, cancelling timer again in __pool_destroy().