xen/cpu: Distinguish "cpu already in that state" in cpu_{up,down}()

xen/cpu: Distinguish "cpu already in that state" in cpu_{up,down}()

All methods of querying the online state of a CPU are racy without the hotplug
lock held, which can lead to a TOCTOU race trying to online or offline CPUs.

Distinguish this case with -EEXIST rather than -EINVAL, so the caller can take
other actions if necessary.

While adjusting this, rework the code slightly to fold the exit paths, which
results in a minor reduction in compiled code size.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>