tsan: Add a missing disable_sanitizer_instrumentation attribute

[glider]

Summary

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().

compiler-rt/test/tsan/Linux/fork_deadlock.cpp

 // RUN: %clangxx_tsan -O1 %s -o %t && %env_tsan_opts=atexit_sleep_ms=0 %run %t 2>&1 | FileCheck %s
 
 // This test models what happens on Mac when fork
 // calls malloc/free inside of our atfork callbacks.
 // and ensures that we don't deadlock on malloc/free calls.
 
 #include "../test.h"
 #include "syscall.h"
 #include <errno.h>
 #include <string.h>
 #include <sys/types.h>
 #include <sys/wait.h>
 
-void alloc_free_blocks() {
+// disable_sanitizer_instrumentation on __tsan_test_only_on_fork is not
+// transitive, so we must apply it here as well.
+// Instrumenting alloc_free_blocks() will result in deadlocks in TSan.
+__attribute__((disable_sanitizer_instrumentation)) void alloc_free_blocks() {
   // Allocate a bunch of blocks to drain local allocator cache
   // and provoke it to lock allocator global mutexes.
   const int kBlocks = 1000;
   void *blocks[kBlocks];
   for (int i = 0; i < kBlocks; i++) {
     void *p = malloc(10);
     *(volatile char *)p = 0;
     blocks[i] = p;