mntns: close_proc before clean up mounts

Because /proc could not be umounted, if any its file is opened.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -688,6 +688,8 @@ int prepare_mnt_ns(int ns_pid)
 
 	pr_info("Restoring mount namespace\n");
 
+	close_proc();
+
 	/*
 	 * The new mount namespace is filled with the mountpoint
 	 * clones from the original one. We have to umount them