cgroup-util: declare file_handle union in cg_cgroupid_open()

This also effectively reverts eb2e91b981,
as now the object is only used as input, and kernel would need to handle
input with only ID no matter whether the struct later gets extended.

src/basic/cgroup-util.c

@@ -35,18 +35,6 @@
 #include "user-util.h"
 #include "xattr-util.h"
 
-/* The structure to pass to name_to_handle_at() on cgroupfs2 */
-typedef union {
-        struct file_handle file_handle;
-        uint8_t space[MAX_HANDLE_SZ];
-} cg_file_handle;
-
-#define CG_FILE_HANDLE_INIT                                     \
-        (cg_file_handle) {                                      \
-                .file_handle.handle_bytes = sizeof(uint64_t),   \
-                .file_handle.handle_type = FILEID_KERNFS,       \
-        }
-
 int cg_is_available(void) {
         struct statfs fs;
 
@@ -82,7 +70,14 @@ int cg_cgroupid_open(int cgroupfs_fd, uint64_t id) {
                 cgroupfs_fd = fsfd;
         }
 
-        cg_file_handle fh = CG_FILE_HANDLE_INIT;
+        union {
+                struct file_handle file_handle;
+                uint8_t space[offsetof(struct file_handle, f_handle) + sizeof(uint64_t)];
+        } fh = {
+                .file_handle.handle_bytes = sizeof(uint64_t),
+                .file_handle.handle_type = FILEID_KERNFS,
+        };
+
         unaligned_write_ne64(fh.file_handle.f_handle, id);
 
         return RET_NERRNO(open_by_handle_at(cgroupfs_fd, &fh.file_handle, O_DIRECTORY|O_CLOEXEC));