We currently mask off offline CPUs in both set_cpus_allowed and
sys_sched_setaffinity.  This is firstly redundant, and secondly
erroneous when more CPUs come online (eg. setting affinity to all 1s
should mean all CPUs, including future ones).

We mask with cpu_online_map() in sys_sched_getaffinity *anyway* (which
is another issue, since this is not valid with changing of online
cpus either), so userspace won't see any difference.

This patch makes set_cpus_allowed() return -errno, and check that in
sys_sched_setaffinity.