mount: use correct mount namespace in open_mountpoint

open_mountpoint() have to operate in target's mount namespace,
which may differ from root's namespace - root_item->pid.real.

So obtain the correct mount namespace from mount_info and use
it in switch_ns().
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -1085,7 +1085,7 @@ static int open_mountpoint(struct mount_info *pm)
 		return -1;
 	}
 
-	if (switch_ns(root_item->pid.real, &mnt_ns_desc, &ns_old) < 0)
+	if (switch_ns(pm->nsid->ns_pid, &mnt_ns_desc, &ns_old) < 0)
 		goto out;
 
 	mnt_path = mkdtemp(mnt_path_tmp);