dir/server/tree: fix missing blockSize
dir/server/tree: fix missing blockSize

CL/7702 missed this one instance.

Tests pass again.

Change-Id: I83574826f2945bcad9267be254f1b35963aafe08
Reviewed-on: https://upspin-review.googlesource.com/7762
Reviewed-by: Eduardo Pinheiro <55040cbb39ba395339ae7940d8da6cd97797a4a8@google.com>

upspin/upspin,upspin/upspin