fix bug: modifying system time(to the past) will cause the flushing-dirty-page thread to suspend

src/gausskernel/process/postmaster/bgwriter.cpp

@@ -748,7 +748,7 @@ static int64 get_bgwriter_sleep_time()
     } else {
         time_diff = 0;
     }
-
+    time_diff = MIN(time_diff, u_sess->attr.attr_storage.BgWriterDelay);
     return time_diff;
 }
 

src/gausskernel/process/postmaster/pagewriter.cpp

@@ -688,6 +688,7 @@ static int64 get_pagewriter_sleep_time()
     } else {
         time_diff = 0;
     }
+    time_diff = MIN(time_diff, u_sess->attr.attr_storage.pageWriterSleep);
     return time_diff;
 }