io_uring/net: add IORING_ACCEPT_DONTWAIT flag

This allows the caller to perform a non-blocking attempt, similarly to
how recvmsg has MSG_DONTWAIT. If set, and we get -EAGAIN on a connection
attempt, propagate the result to userspace rather than arm poll and
wait for a retry.

Suggested-by: Norman Maurer <norman_maurer@apple.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -379,6 +379,7 @@ enum io_uring_op {
  * accept flags stored in sqe->ioprio
  */
 #define IORING_ACCEPT_MULTISHOT	(1U << 0)
+#define IORING_ACCEPT_DONTWAIT	(1U << 1)
 
 /*
  * IORING_OP_MSG_RING command types, stored in sqe->addr