MDEV-26017: Assertion stat.flush_list_bytes <= curr_pool_size

buf_flush_relocate_on_flush_list(): If we are removing the block from
buf_pool.flush_list, subtract its size from buf_pool.stat.flush_list_bytes.
This fixes a regression that was introduced in
commit 22b62eda (MDEV-25113).

storage/innobase/buf/buf0flu.cc

@@ -321,6 +321,7 @@ buf_flush_relocate_on_flush_list(
 	}
 
 	if (lsn == 1) {
+		buf_pool.stat.flush_list_bytes -= bpage->physical_size();
 was_clean:
 		dpage->list.prev = nullptr;
 		dpage->list.next = nullptr;