mm: fix expression to determine which vma-s can be shared

Currently only addresses are compared. It's obviously not enough.

* First of all the parent vma must be private.
* Both vma-s must have the identical set of MAP_GROWSDOWN and MAP_FILES
  flags.
* Both vma-s must be linked to the same file.

https://bugzilla.openvz.org/show_bug.cgi?id=2824


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