btrfs: use integrated bitmaps for btrfs_raid_bio::dbitmap and finish_pbitmap

Previsouly we use "unsigned long *" for those two bitmaps.

But since we only support fixed stripe length (64KiB, already checked in
tree-checker), "unsigned long *" is really a waste of memory, while we
can just use "unsigned long".

This saves us 8 bytes in total for btrfs_raid_bio.

To be extra safe, add an ASSERT() making sure calculated
@stripe_nsectors is always smaller than BITS_PER_LONG.

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