io_uring: NULL files dereference by SQPOLL

SQPOLL task may find sqo_task->files == NULL and
__io_sq_thread_acquire_files() would leave it unset, so following
fget_many() and others try to dereference NULL and fault. Propagate
an error files are missing.

[  118.962785] BUG: kernel NULL pointer dereference, address:
	0000000000000020
[  118.963812] #PF: supervisor read access in kernel mode
[  118.964534] #PF: error_code(0x0000) - not-present page
[  118.969029] RIP: 0010:__fget_files+0xb/0x80
[  119.005409] Call Trace:
[  119.005651]  fget_many+0x2b/0x30
[  119.005964]  io_file_get+0xcf/0x180
[  119.006315]  io_submit_sqes+0x3a4/0x950
[  119.007481]  io_sq_thread+0x1de/0x6a0
[  119.007828]  kthread+0x114/0x150
[  119.008963]  ret_from_fork+0x22/0x30

Reported-by: Josef Grieb <josef.grieb@gmail.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>