Revert "mm/rmap: Fix anon_vma->degree ambiguity leading to double-reuse"

This reverts commit 6dbfc25d which is
commit 2555283eb40df89945557273121e9393ef9b542b upstream.

It currently breaks the Android kernel ABI.  If it needs to come back,
it should be done in an ABI-safe way.

Bug: 161946584
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I46a7a4ffc5d2725146787ea7273a42a5cf062ed4