mount: Core for creating mount namespace

Namespace restore goes in two steps -- first, clean old mountpoints
(except root) that are left after clone(CLONE_NEWNS) call. Next,
repopulate the namespace with new mountpoints (except root) read
from the image file.

This patch adds the skeleton for the above -- reading the mountpoints
from current proc and image and calling for mount/umount. The real
action will come in further patches.

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

namespaces.c

@@ -124,6 +124,8 @@ int prepare_namespace(int pid, unsigned long clone_flags)
 		ret = prepare_utsns(pid);
 	if (clone_flags & CLONE_NEWIPC)
 		ret = prepare_ipc_ns(pid);
+	if (clone_flags & CLONE_NEWNS)
+		ret = prepare_mnt_ns(pid);
 
 	return ret;
 }