A lot of intricacies go into updating the stats when adding or removing
mappings: which stat index to use and which function.  Abstract this away
into a new static helper in rmap.c, __folio_mod_stat().

This adds an unnecessary call to folio_test_anon() in
__folio_add_anon_rmap() and __folio_add_file_rmap().  However, the folio
struct should already be in the cache at this point, so it shouldn't cause
any noticeable overhead.

No functional change intended.

[hughd@google.com: fix /proc/meminfo]
  Link: https://lkml.kernel.org/r/49914517-dfc7-e784-fde0-0e08fafbecc2@google.com
Link: https://lkml.kernel.org/r/20240506211333.346605-1-yosryahmed@google.comSigned-off-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>