{pidfd,cgroup}-util: avoid alignment warning when accessing f_handle (#37520)

NetworkManager imports some systemd code, which we then build with
`-Wcast-align=strict`. This results in this warning:

```
../src/libnm-systemd-shared/src/basic/pidfd-util.c: In function 'pidfd_get_inode_id':
../src/libnm-systemd-shared/src/basic/pidfd-util.c:255:41: warning: cast increases required alignment of target type [-Wcast-align]
  255 |                                 *ret = *(uint64_t*) fh.file_handle.f_handle;
      |                                         ^
```

Since the address of `f_handle` should always be aligned at the word
boundary (it is preceded by 2 int`s` in `file_handle`), this patch uses
`CAST_ALIGN_PTR()` to avoid this warning.

src/basic/cgroup-util.h

@@ -332,4 +332,4 @@ typedef union {
                 .file_handle.handle_type = FILEID_KERNFS,       \
         }
 
-#define CG_FILE_HANDLE_CGROUPID(fh) (*(uint64_t*) (fh).file_handle.f_handle)
+#define CG_FILE_HANDLE_CGROUPID(fh) (*CAST_ALIGN_PTR(uint64_t, (fh).file_handle.f_handle))

src/basic/pidfd-util.c

@@ -248,7 +248,7 @@ int pidfd_get_inode_id(int fd, uint64_t *ret) {
                 r = RET_NERRNO(name_to_handle_at(fd, "", &fh.file_handle, &mnt_id, AT_EMPTY_PATH));
                 if (r >= 0) {
                         if (ret)
-                                *ret = *(uint64_t*) fh.file_handle.f_handle;
+                                *ret = *CAST_ALIGN_PTR(uint64_t, fh.file_handle.f_handle);
                         return 0;
                 }
                 assert(r != -EOVERFLOW);