btrfs: return void from csum_tree_block

Now that csum_tree_block is not returning any errors, we can make
csum_tree_block return void and simplify callers.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>