MDEV-34543 Shutdown hangs while freeing the asynchronous i/o slots

Problem:
========
- During shutdown, InnoDB tries to free the asynchronous
I/O slots and hangs. The reason is that InnoDB disables
asynchronous I/O before waiting for pending
asynchronous I/O to finish.

buf_load(): InnoDB aborts the buffer pool load due to
user requested shutdown and doesn't wait for the asynchronous
read to get completed. This could lead to debug assertion
in buf_flush_buffer_pool() during shutdown

Fix:
===
os_aio_free(): Should wait all read_slots and write_slots
to finish before disabling the aio.

buf_load(): Should wait for pending read request to complete
even though it was aborted.

storage/innobase/buf/buf0dump.cc

@@ -643,9 +643,7 @@ buf_load()
 
 	ut_free(dump);
 
-	if (i == dump_n) {
-		os_aio_wait_until_no_pending_reads();
-	}
+	os_aio_wait_until_no_pending_reads();
 
 	ut_sprintf_timestamp(now);
 

storage/innobase/os/os0file.cc

@@ -3752,11 +3752,11 @@ int os_aio_init()
 
 void os_aio_free()
 {
-  srv_thread_pool->disable_aio();
   delete read_slots;
   delete write_slots;
   read_slots= nullptr;
   write_slots= nullptr;
+  srv_thread_pool->disable_aio();
 }
 
 /** Wait until there are no pending asynchronous writes. */