rename get_buffer to poll_get_buffer for consistency

Function returning `std::task::Poll` enums are usually called `poll_*`
to tell them apart from their async variants.

glommio/src/io/dma_file_stream.rs

@@ -626,7 +626,7 @@ impl DmaStreamReader {
         Poll::Ready(Ok(x))
     }
 
-    fn get_buffer(
+    fn poll_get_buffer(
         &mut self,
         cx: &mut Context<'_>,
         len: u64,