btrfs: raid56: make alloc_rbio_essential_pages() subpage compatible

The non-compatible part is only the bitmap iteration part, now the
bitmap size is extended to rbio::stripe_nsectors, not the old
rbio::stripe_npages.

Since we're here, also slightly improve the function by:

- Rename @i to @stripe
- Rename @bit to @sectornr
- Move @page and @index into the inner loop
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/raid56.c

@@ -2415,14 +2415,16 @@ void raid56_add_scrub_pages(struct btrfs_raid_bio *rbio, struct page *page,
  */
 static int alloc_rbio_essential_pages(struct btrfs_raid_bio *rbio)
 {
-	int i;
-	int bit;
-	int index;
-	struct page *page;
+	const u32 sectorsize = rbio->bioc->fs_info->sectorsize;
+	int stripe;
+	int sectornr;
+
+	for_each_set_bit(sectornr, rbio->dbitmap, rbio->stripe_nsectors) {
+		for (stripe = 0; stripe < rbio->real_stripes; stripe++) {
+			struct page *page;
+			int index = (stripe * rbio->stripe_nsectors + sectornr) *
+				    sectorsize >> PAGE_SHIFT;
 
-	for_each_set_bit(bit, rbio->dbitmap, rbio->stripe_npages) {
-		for (i = 0; i < rbio->real_stripes; i++) {
-			index = i * rbio->stripe_npages + bit;
 			if (rbio->stripe_pages[index])
 				continue;