enforce max_pos stream config in poll_get_buffer_aligned

We need to make sure the stream can't return any bytes beyond the
`max_pos` offset. Right now, `poll_read` respects this while
`poll_get_buffer_aligned` doesn't. This commit makes sure we are
consistent across the two.

glommio/src/io/dma_file_stream.rs

@@ -566,12 +566,18 @@ impl DmaStreamReader {
     pub fn poll_get_buffer_aligned(
         &mut self,
         cx: &mut Context<'_>,
-        len: u64,
+        mut len: u64,
     ) -> Poll<Result<(ReadResult, u64)>> {
         let (start_id, buffer_len, remaining) = {
             let state = self.state.borrow();
             let start_id = state.buffer_id(self.current_pos);
             let offset = state.offset_of(self.current_pos);
+
+            // enforce max_pos
+            if self.current_pos + len > state.max_pos {
+                len = state.max_pos - self.current_pos;
+            }
+
             let buffer_len = (self.buffer_size - offset as u64).min(len);
             (start_id, buffer_len, len - buffer_len)
         };