Fix SMP boot for kernels without CONFIG_HOTPLUG_CPU.

Fix SMP boot for kernels without CONFIG_HOTPLUG_CPU.

The function cpu_is_allowed() appears that it should return a boolean
but in fact returns zero on success and an errno on failure. Thus
rename it to the more vague cpu_up_check(), in line with its return
value and thge fact it has unspecified side effects.

Fix the macro definition of cpu_up_check() when building non-hotplug
kernel to return zero rather than one (which always failed secondary
cpu bringup!).

Signed-off-by: Keir Fraser <keir@xensource.com>