kexec: export free_huge_page to VMCOREINFO

PG_head_mask was added into VMCOREINFO to filter huge pages in b3acc56bfe
("kexec: save PG_head_mask in VMCOREINFO"), but makedumpfile still need
another symbol to filter *hugetlbfs* pages.

If a user hope to filter user pages, makedumpfile tries to exclude them by
checking the condition whether the page is anonymous, but hugetlbfs pages
aren't anonymous while they also be user pages.

We know it's possible to detect them in the same way as PageHuge(),
so we need the start address of free_huge_page():

    int PageHuge(struct page *page)
    {
            if (!PageCompound(page))
                    return 0;

            page = compound_head(page);
            return get_compound_page_dtor(page) == free_huge_page;
    }

For that reason, this patch changes free_huge_page() into public
to export it to VMCOREINFO.

Signed-off-by: Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/hugetlb.c

@@ -856,7 +856,7 @@ struct hstate *size_to_hstate(unsigned long size)
 	return NULL;
 }
 
-static void free_huge_page(struct page *page)
+void free_huge_page(struct page *page)
 {
 	/*
 	 * Can't pass hstate in here because it is called from the