valgrind: ignore tcmalloc uninitialized memory

This is the main source of noise when running valgrind +
tcmalloc.  Apparently there are other issues, so I think we
still need the notcmalloc gitbuilder, but this gets us part of
the way.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #175 from ceph/wip-tcmalloc

valgrind: ignore tcmalloc uninitialized memory