The m->bind will be used to get the "lowest" mount. A mount is
called "lowest" in the chain of bind-mounts when its m->bind is
NULL. I.e. -- it's the mount of the root dir of an FS, all the
others are bind-mounted from it.

The propagate_siblings() is called for each new mount and sets
the bind field for others. A part of these other mounts can be
already mounted.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>