mem: Don't track memory changes if --track-mem is not specified

We have a big mistake in how we track for ptes to be SOFT_DIRTY -- no
need in these checks if the --track-mem is not given.

While fixing this, remember proper checks for the kernel memory tracker.

Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reported-by: Tim Schürmann <info@tim-schuermann.de>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>