kbfs_ops_concur_test: fix numBlocks race in ParallelBlocksCanceled
kbfs_ops_concur_test: fix numBlocks race in ParallelBlocksCanceled

If we call numBlocks after canceling the SyncAll, we race with the
cleanup code that deletes the blocks we just put.  Instead, get the
numBlocks before the cancel.

Issue: KBFS-2131

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