selftests/bpf: Fix double thread join in uprobe_multi_test

ASAN reported a "joining already joined thread" error. The
release_child() may be called multiple times for the same struct
child.

Fix by resetting child->thread to 0 after pthread_join.

Also memset(0) static child variable in test_attach_api().

Acked-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260223190736.649171-15-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: theihor

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

@@ -62,8 +62,10 @@ static void release_child(struct child *child)
 		return;
 	close(child->go[1]);
 	close(child->go[0]);
-	if (child->thread)
+	if (child->thread) {
 		pthread_join(child->thread, NULL);
+		child->thread = 0;
+	}
 	close(child->c2p[0]);
 	close(child->c2p[1]);
 	if (child->pid > 0)
@@ -331,6 +333,8 @@ test_attach_api(const char *binary, const char *pattern, struct bpf_uprobe_multi
 {
 	static struct child child;
 
+	memset(&child, 0, sizeof(child));
+
 	/* no pid filter */
 	__test_attach_api(binary, pattern, opts, NULL);