fork1: Initialize p_ktr earlier

[des]

Summary

We check that p_ktr is empty in proc_dtor(), so we must take care to
initialize it before we do anything that can lead to the failure path.

PR: 291470
MFC after: 1 week

[kib]

IMO it would be more proper to move the initialization into proc_init().

[kib]

In D54215#1238587, @kib wrote:

IMO it would be more proper to move the initialization into proc_init().

And then proc_dtor could assert that p->p_ktr is empty unconditionally, not only when there is a thread owned by the process.