x86: Eliminate pointless adjustment attempts in fixup_irqs()

Not only when an IRQ's affinity equals cpu_online_mask is there
no need to actually try to adjust the affinity, but also when
it's a subset thereof. This particularly avoids adjustment
attempts during system shutdown to any IRQs bound to CPU#0.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Gary Hade <garyhade@us.ibm.com>
LKML-Reference: <4D5D52C2020000780003272C@vpn.id2.novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>