zram: fix broken page writeback

commit 0d83596 ("zram: support page writeback") introduced two
problems.  It overwrites writeback_store's return value as kstrtol's
return value, which makes return value zero so user could see zero as
return value of write syscall even though it wrote data successfully.

It also breaks index value in the loop in that it doesn't increase the
index any longer.  It means it can write only first starting block index
so user couldn't write all idle pages in the zram so lose memory saving
chance.

This patch fixes those issues.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0d83596("zram: support page writeback")
Signed-off-by: Minchan Kim <[email protected]>
Reported-by: Amos Bianchi <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: John Dias <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: minchank

drivers/block/zram/zram_drv.c

@@ -638,8 +638,8 @@ static ssize_t writeback_store(struct device *dev,
 		if (strncmp(buf, PAGE_WB_SIG, sizeof(PAGE_WB_SIG) - 1))
 			return -EINVAL;
 
-		ret = kstrtol(buf + sizeof(PAGE_WB_SIG) - 1, 10, &index);
-		if (ret || index >= nr_pages)
+		if (kstrtol(buf + sizeof(PAGE_WB_SIG) - 1, 10, &index) ||
+				index >= nr_pages)
 			return -EINVAL;
 
 		nr_pages = 1;
@@ -663,7 +663,7 @@ static ssize_t writeback_store(struct device *dev,
 		goto release_init_lock;
 	}
 
-	while (nr_pages--) {
+	for (; nr_pages != 0; index++, nr_pages--) {
 		struct bio_vec bvec;
 
 		bvec.bv_page = page;