restore: set buildinfo_tree for each mount namespace

On restore all namespaces are restored as one tree and
a pointer on the root of this tree will be set on each namespace.

Below you can find a call trace, where it's used:
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1672,6 +1672,7 @@ static int prepare_roots_yard(void)
 static int populate_mnt_ns(int ns_pid, struct mount_info *mis)
 {
 	struct mount_info *pms;
+	struct ns_id *nsid;
 
 	mntinfo_tree = NULL;
 	mntinfo = mis;
@@ -1683,6 +1684,13 @@ static int populate_mnt_ns(int ns_pid, struct mount_info *mis)
 	if (!pms)
 		return -1;
 
+	for (nsid = ns_ids; nsid; nsid = nsid->next) {
+		if (nsid->nd != &mnt_ns_desc)
+			continue;
+
+		nsid->mnt.mntinfo_tree = pms;
+	}
+
 	if (validate_mounts(mis, false))
 		return -1;