sched/headers: Move softlockup detector watchdog methods to <linux/nmi.h>

These methods don't belong into <linux/sched.h>, they are neither directly
related to task_struct or are scheduler functionality.

Put them next to the other watchdog methods in <linux/nmi.h>.

( Arguably that header's name is a misnomer, and this patch makes it
  more so - but it should be renamed in another patch. )

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>