io_uring: fix uring_flush in exit_files() warning

io_uring: fix uring_flush in exit_files() warning

WARNING: CPU: 1 PID: 11100 at fs/io_uring.c:9096
	io_uring_flush+0x326/0x3a0 fs/io_uring.c:9096
RIP: 0010:io_uring_flush+0x326/0x3a0 fs/io_uring.c:9096
Call Trace:
 filp_close+0xb4/0x170 fs/open.c:1280
 close_files fs/file.c:401 [inline]
 put_files_struct fs/file.c:416 [inline]
 put_files_struct+0x1cc/0x350 fs/file.c:413
 exit_files+0x7e/0xa0 fs/file.c:433
 do_exit+0xc22/0x2ae0 kernel/exit.c:820
 do_group_exit+0x125/0x310 kernel/exit.c:922
 get_signal+0x3e9/0x20a0 kernel/signal.c:2770
 arch_do_signal_or_restart+0x2a8/0x1eb0 arch/x86/kernel/signal.c:811
 handle_signal_work kernel/entry/common.c:147 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:171 [inline]
 exit_to_user_mode_prepare+0x148/0x250 kernel/entry/common.c:201
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x19/0x50 kernel/entry/common.c:302
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

An SQPOLL ring creator task may have gotten rid of its file note during
exit and called io_disable_sqo_submit(), but the io_uring is still left
referenced through fdtable, which will be put during close_files() and
cause a false positive warning.

First split the warning into two for more clarity when is hit, and the
add sqo_dead check to handle the described case.

Reported-by: syzbot+a32b546d58dde07875a1@syzkaller.appspotmail.com
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>