btrfs: raid56: move sector_ptr::uptodate into a dedicated bitmap

btrfs: raid56: move sector_ptr::uptodate into a dedicated bitmap

The uptodate boolean member can be extracted into a bitmap, which will
save us some space (1 bit in a byte vs 8 bits in a byte).

Furthermore we do not need to record the uptodate bitmap for bio
sectors, as if bio_sectors[].paddr is valid it means there is a bio and
will be uptodate.

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