folder_block_ops: more consistent shadowing in GetBlocksForReading
folder_block_ops: more consistent shadowing in GetBlocksForReading

Suggested by @jzila.

Issue: #580

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