net: restore macvlan by the same way with and without userns

If userns_restore_one_link() is called outside of usernsd,
it switches into the criu namespace and switches back before exiting.

v2: rid of the include of linux/net_namespace.h in criu/include/net.h,
    as well as the associated defines and feature checks

travis-ci: success for net: simplify restore of macvlan-s (rev2)
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>