restore: Don't mess with last_pid when restoring pidns init

When we fork a pidns init there's no need in specifying its pid,
as it will be autogenerated to 1. Clean the code not to mess with
the last_pid sysctl at all in that case, rather than just omitting
the write into it.

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