By calling send_sig do_SAK is recursively taking the
tasklist_lock, which is silly.

In addition I just audited the kernel and this was the only
place where tasklist_lock is taken inside of task_lock.

So this one line change is a general worthwhile cleanup and
it increases our options on how to fix the ptrace_attach races.
Signed-off-by: Linus Torvalds <torvalds@osdl.org>