This is just like my previous removal of tasklist_lock from first_tgid, and
next_tgid.  It simply had to wait until it was rcu safe to walk the thread
list.

This should be the last instance of the tasklist_lock in proc.  So user
processes should not be able to influence the tasklist lock hold times.
Signed-off-by: NEric W. Biederman <ebiederm@xmission.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>