MFC r292440:

MFC r292440:

proc: fix a race which could result in dereference of bad p_pgrp pointer on
fork

During fork p_starcopy - p_endcopy area of a process is populated with bcopy
with only proc lock held. Another forking thread can find such a process and
proceed to access p_pgrp included in said area.

Fix the problem by moving the field outside. It is being properly assigned
later.

git-svn-id: svn://svn.freebsd.org/base/stable/10@293314 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f