graph: Fix a compiler warning

The calloc() takes the first argument for the number of element, not the
size of the element.  Recent compilers warn about it like below:

    CC       utils/graph.ot
  In file included from /home/namhyung/project/uftrace/utils/symbol.h:18,
                   from /home/namhyung/project/uftrace/uftrace.h:14,
                   from /home/namhyung/project/uftrace/utils/graph.h:7,
                   from /home/namhyung/project/uftrace/utils/graph.c:1:
  /home/namhyung/project/uftrace/utils/graph.c: In function ‘setup_fstack_and_graph’:
  /home/namhyung/project/uftrace/utils/graph.c:300:41: warning: ‘calloc’ sizes specified with ‘sizeof’
         in the earlier argument and not in the later argument [-Wcalloc-transposed-args]
    300 |         task.func_stack = xcalloc(sizeof(*task.func_stack), len);
        |                                         ^
  /home/namhyung/project/uftrace/utils/utils.h:202:38: note: in definition of macro ‘xcalloc’
    202 |                 void *__ptr = calloc(n, sz);                                                       \
        |                                      ^
  /home/namhyung/project/uftrace/utils/graph.c:300:41: note: earlier argument should specify number of
         elements, later size of each element
    300 |         task.func_stack = xcalloc(sizeof(*task.func_stack), len);
        |                                         ^
  /home/namhyung/project/uftrace/utils/utils.h:202:38: note: in definition of macro ‘xcalloc’
    202 |                 void *__ptr = calloc(n, sz);                                                       \
        |                                      ^

Signed-off-by: Namhyung Kim <namhyung@gmail.com>

utils/graph.c

@@ -297,7 +297,7 @@ static void setup_fstack_and_graph(struct uftrace_graph *graph, struct test_data
 	tg->new_sess = true;
 
 	/* TODO: de-couple graph from fstack */
-	task.func_stack = xcalloc(sizeof(*task.func_stack), len);
+	task.func_stack = xcalloc(len, sizeof(*task.func_stack));
 
 	for (i = 0; i < len; i++) {
 		struct uftrace_fstack *fstack = NULL;