x86/smpboot: Sanitize uniprocessor init

The UP related setups for local apic are mangled into smp_sanity_check().

That results in duplicate calls to disable_smp() and makes the code
hard to follow. Let smp_sanity_check() return dedicated values for the
various exit reasons and handle them at the call site.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Jiang Liu <jiang.liu@linux.intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/20150115211703.987833932@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>