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

This reverts commit 02333ac1 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: Ie755775e0b743e488f2ddf32c7a512cab2c07e7d
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>