selftest/futex: Make the error check more precise for futex_numa_mpol

Instead of just checking if the syscall failed as expected, check as
well if the returned error code matches the expected error code.

[ bigeasy: reword the commmit message ]

Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman Long <longman@redhat.com>