ptrace: clear TIF_SYSCALL_TRACE on ptrace detach

On __ptrace_detach(), called from do_exit()->exit_notify()->
forget_original_parent()->exit_ptrace(), the TIF_SYSCALL_TRACE in
thread->flags of the tracee is not cleared up.  This results in the
tracehook_report_syscall_* being called (though there's no longer a tracer
listening to that) upon its further syscalls.

Example scenario - attach "strace" to a running process and kill it (the
strace) with SIGKILL.  You'll see that the syscall trace hooks are still
being called.

The clearing of this flag should be moved from ptrace_detach() to
__ptrace_detach().

Link: http://lkml.kernel.org/r/1472759493-20554-1-git-send-email-alnovak@suse.cz
Signed-off-by: Ales Novak <alnovak@suse.cz>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/ptrace.c

@@ -73,6 +73,8 @@ void __ptrace_unlink(struct task_struct *child)
 {
 	BUG_ON(!child->ptrace);
 
+	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
+
 	child->parent = child->real_parent;
 	list_del_init(&child->ptrace_entry);
 
@@ -489,7 +491,6 @@ static int ptrace_detach(struct task_struct *child, unsigned int data)
 
 	/* Architecture-specific hardware disable .. */
 	ptrace_disable(child);
-	clear_tsk_thread_flag(child, TIF_SYSCALL_TRACE);
 
 	write_lock_irq(&tasklist_lock);
 	/*