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>