io_uring: avoid io-wq -EAGAIN looping for !IOPOLL

If an opcode handler semi-reliably returns -EAGAIN, io_wq_submit_work()
might continue busily hammer the same handler over and over again, which
is not ideal. The -EAGAIN handling in question was put there only for
IOPOLL, so restrict it to IOPOLL mode only where there is no other
recourse than to retry as we cannot wait.

Fixes: def596e9 ("io_uring: support for IO polling")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f168b4f24181942f3614dd8ff648221736f572e6.1652433740.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7573,6 +7573,8 @@ static void io_wq_submit_work(struct io_wq_work *work)
 		 * wait for request slots on the block side.
 		 */
 		if (!needs_poll) {
+			if (!(req->ctx->flags & IORING_SETUP_IOPOLL))
+				break;
 			cond_resched();
 			continue;
 		}