shmctl01: don't use hardcoded index == 0 for SHM_STAT test

Test fails on SHM_STAT testcase:
   shmctl01    5  TFAIL  :  shmctl01.c:173: shmctl01 call failed
                            errno = 22 : Invalid argument
   shmctl(0, SHM_STAT, 0x601060)           = -EINVAL

since following commit:
   commit 99db46ea292780cd978d56932d9445b1e8bdafe8
   Author: Manfred Spraul <manfred@colorfullife.com>
   Date:   Tue May 14 15:46:36 2019 -0700
     ipc: do cyclic id allocation for the ipc object.

Don't rely on index 0 being always available, but instead
use (maximum) index returned by SHM_INFO.

Signed-off-by: Jan Stancek <jstancek@redhat.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Acked-by: Cyril Hrubis <chrubis@suse.cz>

testcases/kernel/syscalls/ipc/shmctl/shmctl01.c

@@ -79,6 +79,7 @@ static void func_info(int ret);
 
 /* Check routine for SHM_STAT */
 static void func_sstat(int ret);
+static void func_sstat_setup(void);
 
 /* Check routine for SHM_LOCK */
 static void func_lock(int ret);
@@ -110,7 +111,7 @@ static struct test_case_t {
 #endif
 	{&shm_id_1, IPC_SET, &buf, func_set, set_setup},
 	{&shm_id_1, IPC_INFO, (struct shmid_ds *) &info, func_info, NULL},
-	{&shm_index, SHM_STAT, &buf, func_sstat, NULL},
+	{&shm_index, SHM_STAT, &buf, func_sstat, func_sstat_setup},
 	{&shm_id_1, SHM_LOCK, NULL, func_lock, NULL},
 	{&shm_id_1, SHM_UNLOCK, NULL, func_unlock, NULL},
 	{&shm_id_1, IPC_RMID, NULL, func_rmid, NULL},
@@ -407,9 +408,23 @@ static void func_info(int ret)
 static void func_sstat(int ret)
 {
 	if (ret >= 0)
-		tst_resm(TPASS, "get correct shared memory id");
+		tst_resm(TPASS, "get correct shared memory id for index: %d",
+			shm_index);
 	else
-		tst_resm(TFAIL, "shared memory id is incorrect");
+		tst_resm(TFAIL, "shared memory id is incorrect, index: %d",
+			shm_index);
+}
+
+static void func_sstat_setup(void)
+{
+	struct shm_info tmp;
+	int ret;
+
+	ret = shmctl(shm_id_1, SHM_INFO, (void *)&tmp);
+	if (ret < 0)
+		tst_resm(TFAIL|TERRNO, "shmctl(SHM_INFO)");
+	else
+		shm_index = ret;
 }
 
 static void func_lock(int ret)