Don't leak MNT_INTERNAL away from internal mounts

We want it only for the stuff created by SB_KERNMOUNT mounts, *not* for
their copies.  As it is, creating a deep stack of bindings of /proc/*/ns/*
somewhere in a new namespace and exiting yields a stack overflow.

Cc: stable@kernel.org
Reported-by: Alexander Aring <aring@mojatatu.com>
Bisected-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Tested-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Tested-by: Alexander Aring <aring@mojatatu.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1089,7 +1089,8 @@ static struct mount *clone_mnt(struct mount *old, struct dentry *root,
 			goto out_free;
 	}
 
-	mnt->mnt.mnt_flags = old->mnt.mnt_flags & ~(MNT_WRITE_HOLD|MNT_MARKED);
+	mnt->mnt.mnt_flags = old->mnt.mnt_flags;
+	mnt->mnt.mnt_flags &= ~(MNT_WRITE_HOLD|MNT_MARKED|MNT_INTERNAL);
 	/* Don't allow unprivileged users to change mount flags */
 	if (flag & CL_UNPRIVILEGED) {
 		mnt->mnt.mnt_flags |= MNT_LOCK_ATIME;