mm: Allow anon exclusive check over hugetlb tail pages

PageAnonExclusive() used to forbid tail pages for hugetlbfs, as that used
to be called mostly in hugetlb specific paths and the head page was
guaranteed.

As we move forward towards merging hugetlb paths into generic mm, we may
start to pass in tail hugetlb pages (when with cont-pte/cont-pmd huge
pages) for such check.  Allow it to properly fetch the head, in which case
the anon-exclusiveness of the head will always represents the tail page.

There's already a sign of it when we look at the fast-gup which already
contain the hugetlb processing altogether: we used to have a specific
commit 5805192c ("mm/gup: handle cont-PTE hugetlb pages correctly in
gup_must_unshare() via GUP-fast") covering that area.  Now with this more
generic change, that can also go away.

Signed-off-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>