rename poll_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(match ready!(self.get_buffer(cx, len, start_id)) {
+        Poll::Ready(match ready!(self.poll_get_buffer(cx, len, start_id)) {
             Ok(x) => {
                 self.skip(len);
                 Ok(x)
@@ -635,7 +635,7 @@ impl DmaStreamReader {
         })
     }
 
-    fn get_buffer(
+    fn poll_get_buffer(
         &mut self,
         cx: &mut Context<'_>,
         len: u64,