btrfs: raid56: introduce btrfs_raid_bio::bio_sectors

This new member is going to fully replace bio_pages in the future, but
for now let's keep them co-exist, until the full switch is done.

Currently cache_rbio_pages() and index_rbio_pages() will also populate
the new array.

And cache_rbio_pages() need to record which sectors are uptodate, so we
also need to introduce sector_ptr::uptodate bit.

To avoid extra memory usage, we let the new @uptodate bit to share bits
with @pgoff.  Now pgoff only has at most 31 bits, which is already more
than enough, as even for 256K page size, we only need 18 bits.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>