futex.h: Use urcu_posix_assert to validate unused values

futex.h: Use urcu_posix_assert to validate unused values

When building on FreeBSD, uaddr2 and val3 are unused. Add a
urcu_posix_assert() to validate that they are zero and hence allow users
of the API to quickly figure out that those are not effectively used.

When building on OpenBSD, val3 is unused. Add a urcu_posix_assert() to
validate that it is zero.

Those asserts are already present in the compat code. Use the same
mechanism to prevent users from expecting futex arguments to be used
when they are in fact discarded.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I4e69d240c6f07da471e6af083854440c060ef53b