s390/gup: fix access_ok() usage in __get_user_pages_fast()

access_ok() returns always "true" on s390. Therefore all access_ok()
invocations are rather pointless.
However when walking page tables we need to make sure that everything
is within bounds of the ASCE limit of the task's address space.
So remove the access_ok() call and add the same check we have in
get_user_pages_fast().
Reviewed-by: NGerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/gup.c

@@ -180,8 +180,7 @@ int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	addr = start;
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
 	end = start + len;
-	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
-					(void __user *)start, len)))
+	if ((end < start) || (end > TASK_SIZE))
 		return 0;
 
 	local_irq_save(flags);