tsan: fix epoll_ctl interceptor

Currently data-race-test unittests fail with the following false positive:


WARNING: ThreadSanitizer: data race (pid=20365)
  Write of size 8 at 0x7da000008050 by thread T54:
    #0 close tsan_interceptors.cc:1483 (racecheck_unittest-linux-amd64-O0+0x0000000eb34a)
    #1 NegativeTests_epoll::Worker2() unittest/posix_tests.cc:1148 (racecheck_unittest-linux-amd64-O0+0x0000000cc6b1)
    #2 MyThread::ThreadBody(MyThread*) unittest/./thread_wrappers_pthread.h:367 (racecheck_unittest-linux-amd64-O0+0x000000097500)

  Previous read of size 8 at 0x7da000008050 by thread T49:
    #0 epoll_ctl tsan_interceptors.cc:1646 (racecheck_unittest-linux-amd64-O0+0x0000000e9fee)
    #1 NegativeTests_epoll::Worker1() unittest/posix_tests.cc:1140 (racecheck_unittest-linux-amd64-O0+0x0000000cc5b5)
    #2 MyThread::ThreadBody(MyThread*) unittest/./thread_wrappers_pthread.h:367 (racecheck_unittest-linux-amd64-O0+0x000000097500)