Second try at flaky futex test.
Second try at flaky futex test.

The flake had the call to futex_unlock_pi() returning EINVAL with the
FUTEX_OWNER_DIED set. In this case, userspace has to clean up stale
state. So instead of calling FUTEX_UNLOCK_PI outright, we'll use the
advised atomic compare_exchange as advised in the man page.

PiperOrigin-RevId: 265163920

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor