selftests/bpf: Fix build error due to certain uninitialized variables

With the latest llvm21 compiler, I hit several errors when building bpf
selftests. Some of errors look like below:

test_maps.c:565:40: error: variable 'val' is uninitialized when passed as a
const pointer argument here [-Werror,-Wuninitialized-const-pointer]
565 | assert(bpf_map_update_elem(fd, NULL, &val, 0) < 0 &&
| ^~~

prog_tests/bpf_iter.c:400:25: error: variable 'c' is uninitialized when passed
as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
400 | write(finish_pipe[1], &c, 1);
| ^

Some other errors have similar the pattern as the above.

These errors are fixed by initializing those variables properly.

Signed-off-by: Yonghong Song <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/prog_tests/bloom_filter_map.c

···
 static void test_fail_cases(void)
 {
 	LIBBPF_OPTS(bpf_map_create_opts, opts);
-	__u32 value;
+	__u32 value = 0;
 	int fd, err;
 
 	/* Invalid key size */

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

···
 static void test_task_sleepable(void)
 {
 	struct bpf_iter_tasks *skel;
-	int pid, status, err, data_pipe[2], finish_pipe[2], c;
+	int pid, status, err, data_pipe[2], finish_pipe[2], c = 0;
 	char *test_data = NULL;
 	char *test_data_long = NULL;
 	char *data[2];

tools/testing/selftests/bpf/prog_tests/uprobe_syscall.c

···
 		.retprobe = true,
 	);
 	struct uprobe_syscall_executed *skel;
-	int pid, status, err, go[2], c;
+	int pid, status, err, go[2], c = 0;
 
 	if (!ASSERT_OK(pipe(go), "pipe"))
 		return;

tools/testing/selftests/bpf/prog_tests/verify_pkcs7_sig.c

···
 	char *tmp_dir;
 	struct test_verify_pkcs7_sig *skel = NULL;
 	struct bpf_map *map;
-	struct data data;
+	struct data data = {};
 	int ret, zero = 0;
 
 	/* Trigger creation of session keyring. */

tools/testing/selftests/bpf/test_maps.c

···
 static void test_queuemap(unsigned int task, void *data)
 {
 	const int MAP_SIZE = 32;
-	__u32 vals[MAP_SIZE + MAP_SIZE/2], val;
+	__u32 vals[MAP_SIZE + MAP_SIZE/2], val = 0;
 	int fd, i;
 
 	/* Fill test values to be used */
···
 static void test_stackmap(unsigned int task, void *data)
 {
 	const int MAP_SIZE = 32;
-	__u32 vals[MAP_SIZE + MAP_SIZE/2], val;
+	__u32 vals[MAP_SIZE + MAP_SIZE/2], val = 0;
 	int fd, i;
 
 	/* Fill test values to be used */