block: fixup plugging stubs for !CONFIG_BLOCK

They used an older prototype, fix it up.
Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

include/linux/blkdev.h

@@ -1297,15 +1297,18 @@ static inline long nr_blockdev_pages(void)
 	return 0;
 }
 
-static inline void blk_start_plug(struct list_head *list)
+struct blk_plug {
+};
+
+static inline void blk_start_plug(struct blk_plug *plug)
 {
 }
 
-static inline void blk_finish_plug(struct list_head *list)
+static inline void blk_finish_plug(struct blk_plug *plug)
 {
 }
 
-static inline void blk_flush_plug(struct task_struct *tsk)
+static inline void blk_flush_plug(struct task_struct *task)
 {
 }