io_uring: Fix use of XArray in __io_uring_files_cancel

We have to drop the lock during each iteration, so there's no advantage
to using the advanced API.  Convert this to a standard xa_for_each() loop.

Reported-by:  <syzbot+27c12725d8ff0bfe1a13@syzkaller.appspotmail.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>