ia64: ptrace_attach_sync_user_rbs: avoid "task->signal != NULL" checks

Preparation to make task->signal immutable, no functional changes.

It doesn't matter which pointer we check under tasklist to ensure the task
was not released, ->signal or ->sighand.  But we are going to make
->signal refcountable, change the code to use ->sighand.

Note: this code doesn't need this check and tasklist_lock at all, it
should be converted to use lock_task_sighand().  And, the code under
SIGNAL_STOP_STOPPED check looks wrong.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/ia64/kernel/ptrace.c

@@ -638,7 +638,7 @@ ptrace_attach_sync_user_rbs (struct task_struct *child)
 	 */
 
 	read_lock(&tasklist_lock);
-	if (child->signal) {
+	if (child->sighand) {
 		spin_lock_irq(&child->sighand->siglock);
 		if (child->state == TASK_STOPPED &&
 		    !test_and_set_tsk_thread_flag(child, TIF_RESTORE_RSE)) {
@@ -662,7 +662,7 @@ ptrace_attach_sync_user_rbs (struct task_struct *child)
 	 * job control stop, so that SIGCONT can be used to wake it up.
 	 */
 	read_lock(&tasklist_lock);
-	if (child->signal) {
+	if (child->sighand) {
 		spin_lock_irq(&child->sighand->siglock);
 		if (child->state == TASK_TRACED &&
 		    (child->signal->flags & SIGNAL_STOP_STOPPED)) {