io_uring: add support for send(2) and recv(2)

This adds IORING_OP_SEND for send(2) support, and IORING_OP_RECV for
recv(2) support.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -90,6 +90,8 @@ enum {
 	IORING_OP_WRITE,
 	IORING_OP_FADVISE,
 	IORING_OP_MADVISE,
+	IORING_OP_SEND,
+	IORING_OP_RECV,
 
 	/* this goes last, obviously */
 	IORING_OP_LAST,