selftests/bpf: signedness bug in enable_all_controllers()

The "len" variable needs to be signed for the error handling to work
properly.

Fixes: 596092ef ("selftests/bpf: enable all available cgroup v2 controllers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/cgroup_helpers.c

@@ -47,7 +47,7 @@ int enable_all_controllers(char *cgroup_path)
 	char buf[PATH_MAX];
 	char *c, *c2;
 	int fd, cfd;
-	size_t len;
+	ssize_t len;
 
 	snprintf(path, sizeof(path), "%s/cgroup.controllers", cgroup_path);
 	fd = open(path, O_RDONLY);