hypervisor call tracepoints hcall_stats touchup.

[Will Schmidt <will_schmidt@vnet.ibm.com>]

The tb_total and purr_total values reported via the hcall_stats code
should be cumulative, rather than being replaced by the latest delta tb
or purr value. 

Tested-by: Will Schmidt <will_schmidt@vnet.ibm.com>
Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>

---
[ This is a touch-up to the "[3/6] powerpc: tracing: Add hypervisor call
tracepoints" patch submitted by Anton a few weeks back, so I've copied
folks Anton had on CC for his original patch, this fix is rather ppc
specific, so can probably go in via the ppc tree, but I've no real
preference. ]

diff --git a/arch/powerpc/platforms/pseries/hvCall_inst.c b/arch/powerpc/platforms/pseries/hvCall_inst.c
index 2f58c71..1fefae7 100644
--- a/arch/powerpc/platforms/pseries/hvCall_inst.c
+++ b/arch/powerpc/platforms/pseries/hvCall_inst.c
@@ -124,8 +124,8 @@ static void probe_hcall_exit(unsigned long opcode, unsigned long retval,
 
 	h = &__get_cpu_var(hcall_stats)[opcode / 4];
 	h->num_calls++;
-	h->tb_total = mftb() - h->tb_start;
-	h->purr_total = mfspr(SPRN_PURR) - h->purr_start;
+	h->tb_total += mftb() - h->tb_start;
+	h->purr_total += mfspr(SPRN_PURR) - h->purr_start;
 
 	put_cpu_var(hcall_stats);
 }