mm: memcg: simplify objcg charge size and stock remainder math

mm: memcg: simplify objcg charge size and stock remainder math

Use PAGE_ALIGN() and a more natural cache remainder calculation.

Link: https://lkml.kernel.org/r/20260302195305.620713-3-hannes@cmpxchg.org
Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
Acked-by: Roman Gushchin <roman.gushchin@linux.dev>
Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Reviewed-by: Hao Li <hao.li@linux.dev>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>