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

This was accidentally removed in #3064.

tokio-util/src/codec/framed_impl.rs

@@ -150,7 +150,7 @@ where
             // got room for at least one byte to read to ensure that we don't
             // get a spurious 0 that looks like EOF
             state.buffer.reserve(1);
-            let bytect = match poll_read_buf(cx, pinned.inner.as_mut(), &mut state.buffer)? {
+            let bytect = match poll_read_buf(pinned.inner.as_mut(), cx, &mut state.buffer)? {
                 Poll::Ready(ct) => ct,
                 Poll::Pending => return Poll::Pending,
             };