Try to restore mounts while a postpone list isn't empty and check
that each iteration has some progress, otherwice it will fails for
preventing infinite loops

v2: rework logic about postpone list
    add more comments

v3: one more attempt to make it more readable
v4: Here is a master class from Pavel how to write self-documented code.
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>