restore: Drop real_pid member from shmem_info structure

It's not needed anymore, it was handing cases
where no fork-with-pid functionality were in
kernel, but now it's simply unneeded.

Also drop redundant getpid() calls.

Passes all tests (except fork test which known to fail).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Acked-by: Pavel Emelyanov <xemul@parllels.com>