btrfs: raid56: cleanup for function __free_raid_bio()

The cleanup involves two things:

- Remove the "__" prefix
  There is no naming confliction.

- Remove the forward declaration
  There is no special function call involved.

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