don't have mounts pin their parents

don't have mounts pin their parents

Simplify the rules for mount refcounts.  Current rules include:
	* being a namespace root => +1
	* being someone's child => +1
	* being someone's child => +1 to parent's refcount, unless you've
				   already been through umount_tree().

The last part is not needed at all.  It makes for more places where need
to decrement refcounts and it creates an asymmetry between the situations
for something that has never been a part of a namespace and something that
left one, both for no good reason.

If mount's refcount has additions from its children, we know that
	* it's either someone's child itself (and will remain so
until umount_tree(), at which point contributions from children
will disappear), or
	* or is the root of namespace (and will remain such until
it either becomes someone's child in another namespace or goes through
umount_tree()), or
	* it is the root of some tree copy, and is currently pinned
by the caller of copy_tree() (and remains such until it either gets
into namespace, or goes to umount_tree()).
In all cases we already have contribution(s) to refcount that will last
as long as the contribution from children remains.  In other words, the
lifetime is not affected by refcount contributions from children.

It might be useful for "is it busy" checks, but those are actually
no harder to express without it.

NB: propagate_mnt_busy() part is an equivalent transformation, ugly as it
is; the current logics is actually wrong and may give false negatives,
but fixing that is for a separate patch (probably earlier in the queue).

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>