io_uring: use correct pointer for io_uring_show_cred()

Previous commit changed how we index the registered credentials, but
neglected to update one spot that is used when the personalities are
iterated through ->show_fdinfo(). Ensure we use the right struct type
for the iteration.

Reported-by:  <syzbot+a6d494688cdb797bdfce@syzkaller.appspotmail.com>
Fixes: 1e6fa521 ("io_uring: COW io_identity on mismatch")
Signed-off-by: Jens Axboe <axboe@kernel.dk>