prevent mount hash conflicts

Currently it's still possible to run into a pathological situation when
two hashed mounts share both parent and mountpoint.  That does not work
well, for obvious reasons.

We are not far from getting rid of that; the only remaining gap is
attach_recursive_mnt() not being careful enough when sliding a tree
under existing mount (for propagated copies or in 'beneath' case for
the original one).

To deal with that cleanly we need to be able to find overmounts
(i.e. mounts on top of parent's root); we could do hash lookups or scan
the list of children but either would be costly.  Since one of the results
we get from that will be prevention of multiple parallel overmounts, let's
just bite the bullet and store a (non-counting) reference to overmount
in struct mount.

With that done, closing the hole in attach_recursive_mnt() becomes easy
- we just need to follow the chain of overmounts before we change the
mountpoint of the mount we are sliding things under.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

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