mm/hugetlb: use helper huge_page_size() to get hugepage size

We can use helper huge_page_size() to get the hugepage size directly to
simplify the code slightly.

[linmiaohe@huawei.com: use helper huge_page_size() to get hugepage size]
  Link: https://lkml.kernel.org/r/20210209021803.49211-1-linmiaohe@huawei.com

Link: https://lkml.kernel.org/r/20210208082450.15716-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>