x86/apic: Use dead_cpu instead of current CPU when cleaning up

x2apic_dead_cpu() cleans up the leftovers of a CPU which got unplugged, but
instead of clearing the dead cpu bit in the cluster mask it clears the
current (alive) cpu bit. Noticed because smp_processor_id() is called in
preemptible code and triggers a debug warning.

[ tglx: Rewrote changelog ]

Fixes: 023a6117 ("x86/apic/x2apic: Simplify cluster management")
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20170926170845.13955-1-bp@alien8.de