fix nmi_watchdog=2 bootup hang

wrmsrl() is broken, dropping the upper 32bits of the value to be
written. This broke the NMI watchdog on AMD hardware. (and it
probably broke other code too.)

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>