s390/gup: handle zero nr_pages case correctly

If [__]get_user_pages_fast() gets called with nr_pages == 0, the current
code would walk the page tables and pin as many pages until the first
invalid pte (or the kernel crashed while writing struct page pointers to
the pages array).
So let's handle at least the nr_pages == 0 case correctly and exit early.
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/gup.c

@@ -180,7 +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 ((end < start) || (end > TASK_SIZE))
+	if ((end <= start) || (end > TASK_SIZE))
 		return 0;
 	/*
 	 * local_irq_save() doesn't prevent pagetable teardown, but does