cgroup-util: enforce alignment of f_handle

cgroup-util: enforce alignment of f_handle

The change in 4d2e61211df7244fae6aaeb8cae0916b4a0e78bd
broke armv7 where uintptr_t is 4 bytes:

/* test_id */
Assertion '((uintptr_t) _p) % alignof(uint64_t) == 0' failed at src/basic/cgroup-util.c:108, function cg_get_cgroupid_at(). Aborting.

(gdb) p (uintptr_t)fh.file_handle.f_handle % _Alignof(uint64_t)
$9 = 4

Enforce that the structure is aligned as expected

Follow-up for 4d2e61211df7244fae6aaeb8cae0916b4a0e78bd