powerpc: Update comments in irqflags.h

The comment on TRACE_ENABLE_INTS is incorrect, and appears to have
always been incorrect since the code was merged. It probably came from
an original out-of-tree patch.

Replace it with something that's correct. Also propagate the message to
RECONCILE_IRQ_STATE(), because it's potentially subtle.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/irqflags.h

@@ -32,9 +32,8 @@
 #endif
 
 /*
- * Most of the CPU's IRQ-state tracing is done from assembly code; we
- * have to call a C function so call a wrapper that saves all the
- * C-clobbered registers.
+ * These are calls to C code, so the caller must be prepared for volatiles to
+ * be clobbered.
  */
 #define TRACE_ENABLE_INTS	TRACE_WITH_FRAME_BUFFER(trace_hardirqs_on)
 #define TRACE_DISABLE_INTS	TRACE_WITH_FRAME_BUFFER(trace_hardirqs_off)
@@ -42,6 +41,9 @@
 /*
  * This is used by assembly code to soft-disable interrupts first and
  * reconcile irq state.
+ *
+ * NB: This may call C code, so the caller must be prepared for volatiles to
+ * be clobbered.
  */
 #define RECONCILE_IRQ_STATE(__rA, __rB)		\
 	lbz	__rA,PACASOFTIRQEN(r13);	\