btrfs: raid56: make btrfs_raid_bio more compact

There are a lot of members using much larger type in btrfs_raid_bio than
necessary, like nr_pages which represents the total number of a full
stripe.

Instead of int (which is at least 32bits), u16 is already enough
(max stripe length will be 256MiB, already beyond current RAID56 device
number limit).

So this patch will reduce the width of the following members:

- stripe_len to u32
- nr_pages to u16
- nr_data to u8
- real_stripes to u8
- scrubp to u8
- faila/b to s8
  As -1 is used to indicate no corruption

This will slightly reduce the size of btrfs_raid_bio from 272 bytes to
256 bytes, reducing 16 bytes usage.

But please note that, when using btrfs_raid_bio, we allocate extra space
for it to cover various pointer array, so the reduce memory is not
really a big saving overall.

As we're here modifying the comments already, update existing comments
to current code standard.

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