f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()

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

f2fs: fix to avoid use-after-free in f2fs_stop_gc_thread()

syzbot reports a f2fs bug as below:

__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
print_report+0xe8/0x550 mm/kasan/report.c:491
kasan_report+0x143/0x180 mm/kasan/report.c:601
kasan_check_range+0x282/0x290 mm/kasan/generic.c:189
instrument_atomic_read_write include/linux/instrumented.h:96 [inline]
atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:252 [inline]
__refcount_add include/linux/refcount.h:184 [inline]
__refcount_inc include/linux/refcount.h:241 [inline]
refcount_inc include/linux/refcount.h:258 [inline]
get_task_struct include/linux/sched/task.h:118 [inline]
kthread_stop+0xca/0x630 kernel/kthread.c:704
f2fs_stop_gc_thread+0x65/0xb0 fs/f2fs/gc.c:210
f2fs_do_shutdown+0x192/0x540 fs/f2fs/file.c:2283
f2fs_ioc_shutdown fs/f2fs/file.c:2325 [inline]
__f2fs_ioctl+0x443a/0xbe60 fs/f2fs/file.c:4325
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:907 [inline]
__se_sys_ioctl+0xfc/0x170 fs/ioctl.c:893
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

The root cause is below race condition, it may cause use-after-free
issue in sbi->gc_th pointer.

- remount
- f2fs_remount
- f2fs_stop_gc_thread
- kfree(gc_th)
- f2fs_ioc_shutdown
- f2fs_do_shutdown
- f2fs_stop_gc_thread
- kthread_stop(gc_th->f2fs_gc_task)
: sbi->gc_thread = NULL;

We will call f2fs_do_shutdown() in two paths:
- for f2fs_ioc_shutdown() path, we should grab sb->s_umount semaphore
for fixing.
- for f2fs_shutdown() path, it's safe since caller has already grabbed
sb->s_umount semaphore.