xa_store() may fail, check the result.

Cc: stable@vger.kernel.org # 5.10
Fixes: 0f212204 ("io_uring: don't rely on weak ->files references")
Signed-off-by: NPavel Begunkov <asml.silence@gmail.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>