mntns: rework validation to support non-root shared bind-mounts (v2)

A problem which is solved in this path is that some children can be
unaccessiable (unvisiable) for non-root bind-mounts

root	mount point
-------------------
/	/a (shared:1)
/	/a/x
/	/a/x/y
/	/a/z
/x	/b (shared:1)
/	/b/y

/b is a non-root bind-mount of /a
/y is visiable to both mounts
/z is vidiable only for /a

Before this patch we checked that the set of children is the same for
all mount in a shared group. Now we check that a visiable set of mounts
is the same for all mounts in a shared group.

Now we take the next mount in the shared group, which is wider or equal
to current and compare children between them.

Before this patch validate_shared(m) validates the m->parent mount.
Now it validates the "m" mount. So you can find following lines in the
patch:
-               if (m->parent->shared_id && validate_shared(m))
+               if (m->shared_id && validate_shared(m))

We doesn't support shared mounts with different set of children.
Here is an example of such case can be created:
mount tmpfs a /a
mount --make-shared /a
mkdir /a/b
mount tmpfs b /a/b
mount --bind /a /c

In this case /c doesn't have the /b child. To support such cases,
we need to sort all shared mounts accoding with a set of children.

v2: If root is equal to "/", its len should be zero. We expect that the
last symbol in a path is not "/".

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>