rst: Create custom proc every time we restore a namespace

We need own proc for restoring mount namespace, since the proc should
be umounted and mounted back diring namespace restore and I don't want
to introduce a special kludge for this.

One more notice -- the temp proc is mounted _after_ namespaces recreation
for the same reason (it will be umounted by prepare_mnt_ns).

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>