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: NMatthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: NJens Axboe <axboe@kernel.dk>