bsplitter: clean up block size logic, suggested by @akalin-keybase
bsplitter: clean up block size logic, suggested by @akalin-keybase

Issue: #443

keybase/kbfs,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/kbfs,keybase/client,keybase/client,keybase/client,keybase/kbfs