perf/arm-cmn: Fix invalid pointer when access dtc object sharing the same IRQ number

When multiple dtcs share the same IRQ number, the irq_friend which
used to refer to dtc object gets calculated incorrect which leads
to invalid pointer.

Fixes: 0ba6477 ("perf: Add Arm CMN-600 PMU driver")

Signed-off-by: Tuan Phan <tuanphan@os.amperecomputing.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/1623946129-3290-1-git-send-email-tuanphan@os.amperecomputing.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: Tuan Phan

drivers/perf/arm-cmn.c

@@ -1212,7 +1212,7 @@ static int arm_cmn_init_irqs(struct arm_cmn *cmn)
 		irq = cmn->dtc[i].irq;
 		for (j = i; j--; ) {
 			if (cmn->dtc[j].irq == irq) {
-				cmn->dtc[j].irq_friend = j - i;
+				cmn->dtc[j].irq_friend = i - j;
 				goto next;
 			}
 		}