In the membuffer cache code, fix the L1->L2 promotion / eviction heuristics
for high hit count entries.  This is an efficiency not a correctness fix.

* subversion/libsvn_subr/cache-membuffer.c
  (ensure_data_insertable_l2): Make sure we use the full 64 bits of the
                               32x32 bit multiplications.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568599 13f79535-47bb-0310-9956-ffa450edef68