mm/rmap: simplify PageAnonExclusive sanity checks when adding anon rmap

Let's sanity-check PageAnonExclusive vs.  mapcount in page_add_anon_rmap()
and hugepage_add_anon_rmap() after setting PageAnonExclusive simply by
re-reading the mapcounts.

We can stop initializing the "first" variable in page_add_anon_rmap() and
no longer need an atomic_inc_and_test() in hugepage_add_anon_rmap().

While at it, switch to VM_WARN_ON_FOLIO().

[david@redhat.com: update check for doubly-mapped page]
  Link: https://lkml.kernel.org/r/d8e5a093-2e22-c14b-7e64-6da280398d9f@redhat.com
Link: https://lkml.kernel.org/r/20230913125113.313322-6-david@redhat.com


Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>