powerpc: Improve behaviour of irq tracing on 64-bit exception entry

Some exceptions would unconditionally disable interrupts on entry,
which is fine, but calling lockdep every time not only adds more
overhead than strictly needed, but also means we get quite a few
"redudant" disable logged, which makes it hard to spot the really
bad ones.

So instead, split the macro used by the exception code into a
normal one and a separate one used when CONFIG_TRACE_IRQFLAGS is
enabled, and make the later skip th tracing if interrupts were
already disabled.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/exception-64s.h

@@ -272,15 +272,34 @@ label##_hv:								\
 	_MASKABLE_EXCEPTION_PSERIES(vec, label,				\
 				    EXC_HV, SOFTEN_TEST_HV)
 
+/*
+ * Our exception common code can be passed various "additions"
+ * to specify the behaviour of interrupts, whether to kick the
+ * runlatch, etc...
+ */
+
+/* Exception addition: Hard disable interrupts */
+#ifdef CONFIG_TRACE_IRQFLAGS
 #define DISABLE_INTS				\
+	lbz	r10,PACASOFTIRQEN(r13);		\
 	li	r11,0;				\
-	stb	r11,PACASOFTIRQEN(r13);		\
+	cmpwi	cr0,r10,0;			\
 	stb	r11,PACAHARDIRQEN(r13);		\
-	TRACE_DISABLE_INTS
+	beq	44f;				\
+	stb	r11,PACASOFTIRQEN(r13);		\
+	TRACE_DISABLE_INTS;			\
+44:
+#else
+#define DISABLE_INTS				\
+	li	r11,0;				\
+	stb	r11,PACASOFTIRQEN(r13);		\
+	stb	r11,PACAHARDIRQEN(r13)
+#endif /* CONFIG_TRACE_IRQFLAGS */
 
+/* Exception addition: Keep interrupt state */
 #define ENABLE_INTS				\
-	ld	r12,_MSR(r1);			\
 	mfmsr	r11;				\
+	ld	r12,_MSR(r1);			\
 	rlwimi	r11,r12,0,MSR_EE;		\
 	mtmsrd	r11,1