watchdog/hardlockup: Provide interface to stop/restart perf events

Provide an interface to stop and restart perf NMI watchdog events on all
CPUs. This is only usable during init and especially for handling the perf
HT bug on Intel machines. It's safe to use it this way as nothing can
start/stop the NMI watchdog in parallel.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Don Zickus <dzickus@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Ulrich Obergfell <uobergfe@redhat.com>
Link: http://lkml.kernel.org/r/20170912194146.167649596@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>