ptrace: ptrace_attach: use send_sig_info() instead force_sig_specific()

Nobody can block/ignore SIGSTOP, no need to use force_sig_specific() in
ptrace_attach.  Use the "regular" send_sig_info().

With this patch stracing of /sbin/init doesn't clear its SIGNAL_UNKILLABLE,
but not that this makes ptracing of init safe.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -208,8 +208,7 @@ int ptrace_attach(struct task_struct *task)
 
 	__ptrace_link(task, current);
 
-	force_sig_specific(SIGSTOP, task);
-
+	send_sig_info(SIGSTOP, SEND_SIG_FORCED, task);
 bad:
 	write_unlock_irqrestore(&tasklist_lock, flags);
 	task_unlock(task);