i386: use the right wrapper to disable the NMI watchdog

When disabled through /proc/sys/kernel/nmi_watchdog, the NMI watchdog uses the
stop() method directly, which does not decrement the activity counter, leading
to a BUG().  Use the wrapper function instead to fix that.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>