tsan: Add a missing disable_sanitizer_instrumentation attribute

Turns out the test was working by accident: we need to ensure
TSan instrumentation is not called from the fork() hook, otherwise the
tool will deadlock. Previously it worked because alloc_free_blocks() got
inlined into __tsan_test_only_on_fork(), but it cannot always be the
case.

Adding __attribute__((disable_sanitizer_instrumentation)) will prevent
TSan from instrumenting alloc_free_blocks().

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D120050