In case if we need to use vdso proxy the memory area
which holds restorer also has a place for vdso proxy
code itself, so on final pass we should not unmap it,
otherwise any call to vdso function will cause sigsegv.

IOW, the memory before final "cleanup" pass of restorer
might look as

    +-----------+---------+     +-------------+------+
    | bootstrap | rt-vdso | ... | application | vdso |
    +-----------+---------+     +-------------+------+
                       ^                         |
                       `-------------------------+

and we have redirected "vdso" code to jump to "rt-vdso".
After final pass the memory must look as

                +---------+     +-------------+------+
                | rt-vdso | ... | application | vdso |
                +---------+     +-------------+------+
                       ^                         |
                       `-------------------------+

I noticed this problem during container migration
testing, the container itself was suspended on 2.6.32
OpenVZ kernel with apache running inside, and any attempt
to connect to apache caused apache to crash.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>