accept02: Join the multicast group on lo

The test joins the multicast group with interface 0. Then the kernel
needs a route to 224.0.0.0 to find a device. When we only have lo,
like in a QEMU guest with no network, there is no such route and the
test breaks:

  accept02.c:52: TBROK: setsockopt(3, 0, 42, ...) failed: ENODEV (19)

All traffic in the test goes over 127.0.0.1, so join the group on lo.

Link: https://lore.kernel.org/ltp/20260928212733.2835408-1-anders.roxell@linaro.org/
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Martin Doucha <mdoucha@suse.cz>
Tested-by: Martin Doucha <mdoucha@suse.cz>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>

testcases/kernel/syscalls/accept/accept02.c

diff --git a/testcases/kernel/syscalls/accept/accept02.c b/testcases/kernel/syscalls/accept/accept02.c
index 1705662..09a1c35 100644
--- a/testcases/kernel/syscalls/accept/accept02.c
+++ b/testcases/kernel/syscalls/accept/accept02.c
@@ -22,6 +22,7 @@
 #include <errno.h>
 #include <sys/socket.h>
 #include "tst_test.h"
+#include "tst_netdevice.h"
 #include "tst_safe_net.h"
 #include "tst_safe_pthread.h"
 
@@ -112,7 +113,7 @@
 	client_addr = tst_alloc(sizeof(*client_addr));
 	mc_group = tst_alloc(sizeof(*mc_group));
 
-	mc_group->gr_interface = 0;
+	mc_group->gr_interface = NETDEV_INDEX_BY_NAME("lo");
 	mc_group_addr = (struct sockaddr_in *) &mc_group->gr_group;
 	mc_group_addr->sin_family = AF_INET;
 	inet_aton(MULTICASTIP, &mc_group_addr->sin_addr);