sparc64: Manage NMI watchdog enabling like x86.

Use a per-cpu 'wd_enabled' boolean and a global atomic_t count
of watchdog NMI enabled cpus which is set to '-1' if something
is wrong with the watchdog and it can't be used.

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/arch/sparc/include/asm/nmi.h b/arch/sparc/include/asm/nmi.h
index fbd546d..c7d11e4 100644
--- a/arch/sparc/include/asm/nmi.h
+++ b/arch/sparc/include/asm/nmi.h
@@ -5,6 +5,6 @@
 extern void perfctr_irq(int irq, struct pt_regs *regs);
 extern void nmi_adjust_hz(unsigned int new_hz);
 
-extern int nmi_usable;
+extern atomic_t nmi_active;
 
 #endif /* __NMI_H */