[tsan] Don't abort when a deadlock detector finds a mutex cycle longer than 10

In one of the already existing apps that I'm testing TSan on, I really see a mutex path that is longer than 10 (but not by much, something like 11-13 actually). Let's raise this to 20 and weaken the assertion so we don't crash.

Differential Revision: http://reviews.llvm.org/D20427

llvm-svn: 270319