MIPS: Compat: Use 32-bit wrapper for compat_sys_futex.

We can't trust userspace to pass signed-extend arguments.  Not correctly
sign-extended arguments to futex-wait result in architecturally undefined
operation of 32-bit arithmetic instructions.

For example, if 'val' is too big and bit-31 is 1, the caller may enter
endless loop at:

futex_wait_setup()
{
	...

	if (uval != val) {
		queue_unlock(q, *hb);
		ret = -EWOULDBLOCK;

	...
}

Signed-off-by: Yong Zhang <yong.zhang@windriver.com>
To: linux-mips@linux-mips.org
To: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/2714/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>