Free metadata_buffer_index after transfer finished (sgl-project#5364)

Author: jokerwyt

python/sglang/srt/disaggregation/prefill.py

@@ -226,6 +226,11 @@ def process_disagg_prefill_inflight_queue(self: Scheduler) -> None:
             elif poll == KVPoll.Failed:
                 raise Exception("Transferring failed")
 
+        for req in done_reqs:
+            self.disagg_prefill_pending_queue.req_to_metadata_buffer_idx_allocator.free(
+                req.metadata_buffer_index
+            )
+
         # Stream requests which have finished transfer
         self.stream_output(done_reqs, False, None)