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

This reverts commit 94b4b41d 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: Ia52587cbd71a9c7220e888fd6725327d953da03f
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>