A small fix for the long-standing ttm vm page protection hack.

Sent as a separate PR as it touches mm, has all acks in place.
Signed-off-by: Dave Airlie <airlied@redhat.com>
From: Thomas Hellström (VMware) <thellstrom@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200116102411.3056-1-thomas_os@shipmail.org