cgroup: Split net_prio.ifpriomap writes

At this moment the kernel doesn't support multi-line write()
to net_prio.ifpriomap. On such write the kernel will parse
only the first line.

That results in the following failed restore:
> (01.454153) cg: Restoring cgroup property value [lo 0
> enp5s0 0
> enp6s0 0
> br0 0
> venet0 0
> host-routed 0
> virbr0 0
> virbr0-nic 0] to [net_prio,net_cls/machine.slice/91d3ac46-380e-4d91-89d3-d2bd1eb30f97/net_prio.ifpriomap]
> (01.454188) Error (criu/cgroup.c:1225): cg: Failed writing lo 0
> enp5s0 0
> enp6s0 0
> br0 0
> venet0 0
> host-routed 0
> virbr0 0
> virbr0-nic 0 to net_prio,net_cls/machine.slice/91d3ac46-380e-4d91-89d3-d2bd1eb30f97/net_prio.ifpriomap: Argument list too long
> (01.472144) Error (criu/cr-restore.c:1013): 20509 killed by signal 9: Killed
> (01.472391) mnt: Switching to new ns to clean ghosts
> (01.472690) Error (criu/cr-restore.c:2007): Restoring FAILED.

Signed-off-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>