mm/memcg: minor cleanup for mc_handle_present_pte()

mm/memcg: minor cleanup for mc_handle_present_pte()

When pagetable lock is held, the page will always be page_mapped().  So
remove unneeded page_mapped() check.  Also the page can't be freed from
under us in this case.  So use get_page() to get extra page reference to
simplify the code.  No functional change intended.

Link: https://lkml.kernel.org/r/20230717113644.3026478-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>