Add explicit comments about purpose of writing to pad

src/core/support/stack_lockfree.c

@@ -100,6 +100,7 @@ gpr_stack_lockfree *gpr_stack_lockfree_create(size_t entries) {
   /* Point the head at reserved dummy entry */
   stack->head.contents.index = INVALID_ENTRY_INDEX;
 #ifdef GPR_ARCH_64
+  // Fill in the pad to avoid confusing memcheck tools
   stack->head.contents.pad = 0;
 #endif
   stack->head.contents.aba_ctr = 0;
@@ -120,6 +121,7 @@ int gpr_stack_lockfree_push(gpr_stack_lockfree *stack, int entry) {
   /* First fill in the entry's index and aba ctr for new head */
   newhead.contents.index = (uint16_t)entry;
 #ifdef GPR_ARCH_64
+  // Fill in the pad to avoid confusing memcheck tools
   newhead.contents.pad = 0;
 #endif