bsplitter_simple: reduce number of maxPtrs to 75%
bsplitter_simple: reduce number of maxPtrs to 75%

Otherwise, with real-life indirect pointers we end up bursting through
the max block size.

Issue: KBFS-35

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