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>