i386: Fix the K7 NMI watchdog checkbit

The performance counters on K7 are only 48 bits wide, so using bit 63 to
check if the counter overflowed is wrong. Let's use bit 47 instead.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Cc: Don Zickus <dzickus@redhat.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>