Fix: futex can be free'd while used by waker thread

Fix: futex can be free'd while used by waker thread

The futex_nto1 utils assume that the futex it operates on
has a program-long lifetime (or that is is protected by a
third-party).

The notification command system uses a futex allocated on the
waiter's stack. However, the waiter could never enter the
futex() syscall (due to of the opportunist check before the futex
call). In this case, the waiter's stack-allocated futex becomes
invalid, but will be used by the waker to perform the FUTEX_WAKE
operation.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>