folder_block_manager: only gc-unref blocks with 0 ref count
folder_block_manager: only gc-unref blocks with 0 ref count

The unref block list in a gcOp is only used by clients to delete
blocks from local caches when the are completely unreferenced in the
TLF.  The bserver tells us when that happens, so only add those blocks
to the gcOp when all references are gone.

Issue: KBFS-900

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