Fix this compile error on s390:
  CC [M]  fs/nfs/blocklayout/blocklayout.o
fs/nfs/blocklayout/blocklayout.c: In function 'bl_end_io_read':
fs/nfs/blocklayout/blocklayout.c:201:4: error: implicit declaration of function 'prefetchw'
Introduced with 
9549ec01 "pnfsblock: bl_read_pagelist".
Cc: Fred Isaman <iisaman@citi.umich.edu>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
 
 #include <linux/namei.h>
 #include <linux/bio.h>         /* struct bio */
 #include <linux/buffer_head.h> /* various write calls */
+#include <linux/prefetch.h>
 
 #include "blocklayout.h"