writeback: fix problem with !CONFIG_BLOCK compilation

When CONFIG_BLOCK isn't enabled:

mm/page-writeback.c: In function 'laptop_mode_timer_fn':
mm/page-writeback.c:708: error: dereferencing pointer to incomplete type
mm/page-writeback.c:709: error: dereferencing pointer to incomplete type

Fix this by essentially eliminating the laptop sync handlers when
CONFIG_BLOCK isn't set, as most are only used from the block layer code.
The exception is laptop_sync_completion() which is used from sys_sync(),
make that an empty declaration in that case.

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

mm/page-writeback.c

@@ -694,6 +694,7 @@ int dirty_writeback_centisecs_handler(ctl_table *table, int write,
 	return 0;
 }
 
+#ifdef CONFIG_BLOCK
 void laptop_mode_timer_fn(unsigned long data)
 {
 	struct request_queue *q = (struct request_queue *)data;
@@ -735,6 +736,7 @@ void laptop_sync_completion(void)
 
 	rcu_read_unlock();
 }
+#endif
 
 /*
  * If ratelimit_pages is too high then we can get into dirty-data overload