Revert "mm: hugetlb: independent PMD page table shared count"

This reverts commit 8410996e which is
commit 59d9094d upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: I23d030d07149986b843bbddc1a62094d90c69957
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>