Break fs/namespace.c so I remember the mount propogation bug that can introduce mount namespace loops.

Currently younger mount namespaces mounted into older mount namespaces are allowed.

Ugh.

unsharing a mount namespace as well as propogating a mount can cause
trouble.

Mount propogation is royal pain to think about and deal with.  I
create mount namespaces so my mounts are different not so my set of
mounts is the same.