util: add back public poll_read_buf() function (#3079)

This was accidentally removed in #3064.

tokio-util/src/io/read_buf.rs

@@ -59,7 +59,7 @@ where
 
         fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
             let this = &mut *self;
-            crate::util::poll_read_buf(cx, Pin::new(this.0), this.1)
+            crate::util::poll_read_buf(Pin::new(this.0), cx, this.1)
         }
     }
 }