proc: allow parse_thread to use an existent buffer

parse_thread allocated a buffer for threads and then it initialized read
pid for each thread.

Now we want to use it on restore and in this moment we already have
a buffer with initialized virt pid-s, so we need to initialize read
pid-s only.

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>