x86/apic: Split out restore_boot_irq_mode() from disable_IO_APIC()

This is a preparation patch. Split out the code which restores boot
irq mode from disable_IO_APIC() into the new restore_boot_irq_mode()
function.

No functional changes.

Signed-off-by: Baoquan He <bhe@redhat.com>
Reviewed-by: Eric W. Biederman <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: douly.fnst@cn.fujitsu.com
Cc: joro@8bytes.org
Cc: prarit@redhat.com
Cc: uobergfe@redhat.com
Link: http://lkml.kernel.org/r/20180214054656.3780-2-bhe@redhat.com


[ Build fix for !CONFIG_IO_APIC and rewrote the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>