io_uring: fix missing ->mm on exit

Upstream commits:

8eb06d7e ("io_uring: fix missing ->mm on exit")
cbcf7214 ("io_uring: return locked and pinned page accounting")

do_exit() first drops current->mm and then runs task_work, from where
io_sq_thread_acquire_mm() would try to set mm for a user dying process.

[  208.004249] WARNING: CPU: 2 PID: 1854 at
	kernel/kthread.c:1238 kthread_use_mm+0x244/0x270
[  208.004287]  kthread_use_mm+0x244/0x270
[  208.004288]  io_sq_thread_acquire_mm.part.0+0x54/0x80
[  208.004290]  io_async_task_func+0x258/0x2ac
[  208.004291]  task_work_run+0xc8/0x210
[  208.004294]  do_exit+0x1b8/0x430
[  208.004295]  do_group_exit+0x44/0xac
[  208.004296]  get_signal+0x164/0x69c
[  208.004298]  do_signal+0x94/0x1d0
[  208.004299]  do_notify_resume+0x18c/0x340
[  208.004300]  work_pending+0x8/0x3d4

Reported-by: Roman Gershman <romange@gmail.com>
Tested-by: Roman Gershman <romange@gmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>