Revert "mm/memremap: fix memunmap_pages() race with get_dev_pagemap()"

This reverts commit d56c5eac which is
commit 1e57ffb6 upstream.

It breaks the Android GKI kernel abi, and is not needed for Android
devices, so revert it for now.  If it is needed for this branch, it can
come back later in an ABI-stable way.

Bug: 161946584
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I1de66e43d8e4356adfec411d477cf4ddde1f6ace