userfaultfd: use vma_pagesize for all huge page size calculation

In __mcopy_atomic_hugetlb() we use two variables to deal with huge page
size: vma_hpagesize and huge_page_size.

Since they are the same, it is not necessary to use two different
mechanism. This patch makes it consistent by all using vma_hpagesize.

Link: http://lkml.kernel.org/r/20190927070032.2129-1-richardw.yang@linux.intel.com
Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>