qxl: fix qxl_release_{map,unmap}

s/PAGE_SIZE/PAGE_MASK/

Luckily release_offset is never larger than PAGE_SIZE, so the bug has no
bad side effects and managed to stay unnoticed for years that way ...

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180418054257.15388-2-kraxel@redhat.com


Signed-off-by: Sean Paul <seanpaul@chromium.org>