This is the latest version of the generic pidhash patch.  The biggest
change is the removal of separately allocated pid structures: they are
now part of the task structure and the first task that uses a PID will
provide the pid structure.  Task refcounting is used to avoid the
freeing of the task structure before every member of a process group or
session has exited.

This approach has a number of advantages besides the performance gains.
Besides simplifying the whole hashing code significantly, attach_pid()
is now fundamentally atomic and can be called during create_process()
without worrying about task-list side-effects.  It does not have to
re-search the pidhash to find out about raced PID-adding either, and
attach_pid() cannot fail due to OOM.  detach_pid() can do a simple
put_task_struct() instead of the kmem_cache_free().

The only minimal downside is the potential pending task structures after
session leaders or group leaders have exited - but the number of orphan
sessions and process groups is usually very low - and even if it's
higher, this can be regarded as a slow execution of the final
deallocation of the session leader, not some additional burden.