btrfs: raid56: avoid double for loop inside alloc_rbio_essential_pages()

The double loop is just checking if the page for the vertical stripe
is allocated.

We can easily convert it to single loop and get rid of @stripe variable.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -2380,24 +2380,23 @@ void raid56_add_scrub_pages(struct btrfs_raid_bio *rbio, struct page *page,
 static int alloc_rbio_essential_pages(struct btrfs_raid_bio *rbio)
 {
 	const u32 sectorsize = rbio->bioc->fs_info->sectorsize;
-	int stripe;
-	int sectornr;
+	int total_sector_nr;
 
-	for_each_set_bit(sectornr, &rbio->dbitmap, rbio->stripe_nsectors) {
-		for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
+	for (total_sector_nr = 0; total_sector_nr < rbio->nr_sectors;
+	     total_sector_nr++) {
 		struct page *page;
-			int index = (stripe * rbio->stripe_nsectors + sectornr) *
-				    sectorsize >> PAGE_SHIFT;
+		int sectornr = total_sector_nr % rbio->stripe_nsectors;
+		int index = (total_sector_nr * sectorsize) >> PAGE_SHIFT;
 
+		if (!test_bit(sectornr, &rbio->dbitmap))
+			continue;
 		if (rbio->stripe_pages[index])
 			continue;
-
 		page = alloc_page(GFP_NOFS);
 		if (!page)
 			return -ENOMEM;
 		rbio->stripe_pages[index] = page;
 	}
-	}
 	index_stripe_sectors(rbio);
 	return 0;
 }