block:fix the comment error in blkdev.h

There is not a function rq_init but blk_rq_init in block/blk-core.c.
Signed-off-by: NWanlong Gao <wanlong.gao@gmail.com>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>

include/linux/blkdev.h

@@ -73,7 +73,7 @@ enum rq_cmd_type_bits {
 
 /*
  * try to put the fields that are referenced together in the same cacheline.
- * if you modify this structure, be sure to check block/blk-core.c:rq_init()
+ * if you modify this structure, be sure to check block/blk-core.c:blk_rq_init()
  * as well!
  */
 struct request {