x86_64 irq: use mask/unmask and proper locking in fixup_irqs()

Force irq migration path during cpu offline, is not using proper locks and
irq_chip mask/unmask routines.  This will result in some races(especially
the device generating the interrupt can see some inconsistent state,
resulting in issues like stuck irq,..).

Appended patch fixes the issue by taking proper lock and encapsulating
irq_chip set_affinity() with a mask() before and an unmask() after.

This fixes a MSI irq stuck issue reported by Darrick Wong.

There are several more general bugs in this area(irq migration in the
process context). For example,

 1. Possibility of missing edge triggered irq.
 2. Reliable method of migrating level triggered irq in the process context.

We plan to look and close these in the near future.

Eric says:
	In addition even with the fix from Suresh there is still at least one
	nasty hardware race in fixup_irqs().   However we exercise that code
	path rarely enough that we are unlikely to hit it in the real world,
	and that race seems to have existed since the code was merged.  And a
	fix for that is not coming soon as it is an open investigation area
	if we can fix irq migration to work outside of irq context or if
	we have to rework the requirements imposed by the generic cpu hotplug
	and layer on fixup_irqs().  So this may come up again.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Reported-and-tested-by: Darrick Wong <djwong@us.ibm.com>
Cc: Andi Kleen <ak@suse.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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