mem: Move shmem preparation routine and rename

We'll collect VmaEntries early before fork.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
rst: Collect VmaEntries only once on restore

Right now we do it two times -- on shmem prepare and
on the restore itself. Make collection only once as
we do for fdinfo-s -- root task reads all stuff in and
populates tasks' rst_info with it.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>