x86, iommu: Mark DMAR IRQ as non-threaded

Mark this lowlevel IRQ handler as non-threaded. This prevents a boot
crash when "threadirqs" is on the kernel commandline. Also the
interrupt handler is handling hardware critical events which should
not be delayed into a thread.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>