Revert "memremap: remove support for external pgmap refcounts"

This reverts commit d18a9085 which is
commit b80892ca 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: Ifc8817c504e48ece90813a42015cb91fc04de5e4