f2fs: fix to invalidate dcc->f2fs_issue_discard in error path

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

f2fs: fix to invalidate dcc->f2fs_issue_discard in error path

Syzbot reports a NULL pointer dereference issue as below:

__refcount_add include/linux/refcount.h:193 [inline]

__refcount_inc include/linux/refcount.h:250 [inline]

refcount_inc include/linux/refcount.h:267 [inline]

get_task_struct include/linux/sched/task.h:110 [inline]

kthread_stop+0x34/0x1c0 kernel/kthread.c:703

f2fs_stop_discard_thread+0x3c/0x5c fs/f2fs/segment.c:1638

kill_f2fs_super+0x5c/0x194 fs/f2fs/super.c:4522

deactivate_locked_super+0x70/0xe8 fs/super.c:332

deactivate_super+0xd0/0xd4 fs/super.c:363

cleanup_mnt+0x1f8/0x234 fs/namespace.c:1186

__cleanup_mnt+0x20/0x30 fs/namespace.c:1193

task_work_run+0xc4/0x14c kernel/task_work.c:177

exit_task_work include/linux/task_work.h:38 [inline]

do_exit+0x26c/0xbe0 kernel/exit.c:795

do_group_exit+0x60/0xe8 kernel/exit.c:925

__do_sys_exit_group kernel/exit.c:936 [inline]

__se_sys_exit_group kernel/exit.c:934 [inline]

__wake_up_parent+0x0/0x40 kernel/exit.c:934

__invoke_syscall arch/arm64/kernel/syscall.c:38 [inline]

invoke_syscall arch/arm64/kernel/syscall.c:52 [inline]

el0_svc_common+0x138/0x220 arch/arm64/kernel/syscall.c:142

do_el0_svc+0x48/0x164 arch/arm64/kernel/syscall.c:206

el0_svc+0x58/0x150 arch/arm64/kernel/entry-common.c:636

el0t_64_sync_handler+0x84/0xf0 arch/arm64/kernel/entry-common.c:654

el0t_64_sync+0x18c/0x190 arch/arm64/kernel/entry.S:581

The root cause of this issue is in error path of f2fs_start_discard_thread(),

it missed to invalidate dcc->f2fs_issue_discard, later kthread_stop() may

access invalid pointer.