parasite: Don't keep code_orig on parasite_ctl

We need this only once -- while calling the mmap from remote
context -- so it's enough to have on-stack variable.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>