Check CPUs online, not configured.

Check CPUs online, not configured.

When the number of CPUs detected is greater than the number of CPUs in
the system, rasdaemon will crash when it receives some events.

Looking deeper, we also fail to use the poll method for similar reasons
in this case.

All of this can be prevented by checking to see how many CPUs are
currently online (sysconf(_SC_NPROCESSORS_ONLN)) instead of how many
CPUs the current kernel was configured to support
(sysconf(_SC_NPROCESSORS_CONF)).

For the kernel side of the discussion, see https://lore.kernel.org/lkml/CAM6Wdxft33zLeeXHhmNX5jyJtfGTLiwkQSApc=10fqf+rQh9DA@mail.gmail.com/T/
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>