x86/nmi: Push duration printk() to irq context

Calling printk() from NMI context is bad (TM), so move it to IRQ
context.

In doing so we slightly change (probably wreck) the debugfs
nmi_longest_ns thingy, in that it doesn't update to reflect the
longest, nor does writing to it reset the count.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-rdw0au56a5ymis1u8p48c12d@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>