Fix missing ret assignment in __bio_map_user() error path

If get_user_pages() returns less pages than what we asked for, we
jump to out_unmap which will return ERR_PTR(ret). But ret can contain
a positive number just smaller than local_nr_pages, so be sure to set
it to -EFAULT always.

Problem found and diagnosed by Damien Le Moal <damien@sdl.hitachi.co.jp>

Signed-off-by: Jens Axboe <axboe@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

fs/bio.c

@@ -655,9 +655,10 @@ static struct bio *__bio_map_user_iov(request_queue_t *q,
 				     write_to_vm, 0, &pages[cur_page], NULL);
 		up_read(&current->mm->mmap_sem);
 
-		if (ret < local_nr_pages)
+		if (ret < local_nr_pages) {
+			ret = -EFAULT;
 			goto out_unmap;
-
+		}
 
 		offset = uaddr & ~PAGE_MASK;
 		for (j = cur_page; j < page_limit; j++) {