taskq_dispatch2: add WARN_ON() if tq || callback is NULL

I see this is happens in gdb:

gdb bt:
 #0  taskq_dispatch2 (tq=0xffff88003712e600, func=0x0 <irq_stack_union>,
     arg=0x0 <irq_stack_union>, flags=0, delay=1)
     at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/taskq.c:263
 dtrace4linux#1  0xffffffffa0369433 in timeout (
     func=func@entry=0xffffffffa035c9d5 <fasttrap_pid_cleanup_cb>,
     arg=arg@entry=0x0 <irq_stack_union>, ticks=ticks@entry=1)
     at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/taskq.c:306
 dtrace4linux#2  0xffffffffa035c9c0 in fasttrap_pid_cleanup ()
     at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/fasttrap.c:505
 dtrace4linux#3  0xffffffffa035cdbf in fasttrap_provider_retire (pid=<optimized out>,
     name=0xffffffffa037eb8f "pid", name@entry=0x140ef <Address 0x140ef out of bounds>,
     mprov=mprov@entry=0) at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/fasttrap.c:1713
 dtrace4linux#4  0xffffffffa035ce08 in fasttrap_exec_exit (p=0xffff8800371d3b18)
     at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/fasttrap.c:596
 dtrace4linux#5  0xffffffffa0358c09 in proc_exit_notifier (n=<optimized out>, code=<optimized out>,
     ptr=<optimized out>)
     at /usr/src/dtrace4linux/build-3.11.0-rc4+/driver/dtrace_linux.c:2020
 dtrace4linux#6  0xffffffff8139b2d3 in notifier_call_chain (
     nl=nl@entry=0xffffffff81634da0 <task_exit_notifier+32>, val=val@entry=0,
     v=v@entry=0xffff88007b62b040, nr_to_call=nr_to_call@entry=-1,
     nr_calls=nr_calls@entry=0x0 <irq_stack_union>) at kernel/notifier.c:93
 dtrace4linux#7  0xffffffff8105824b in __blocking_notifier_call_chain (
     nh=nh@entry=0xffffffff81634d80 <task_exit_notifier>, val=val@entry=0,
     v=0xffff88007b62b040, nr_to_call=nr_to_call@entry=-1,
     nr_calls=nr_calls@entry=0x0 <irq_stack_union>) at kernel/notifier.c:314
 dtrace4linux#8  0xffffffff81058273 in blocking_notifier_call_chain (
     nh=nh@entry=0xffffffff81634d80 <task_exit_notifier>, val=val@entry=0,
     v=<optimized out>) at kernel/notifier.c:325
 dtrace4linux#9  0xffffffff810750f5 in profile_task_exit (task=<optimized out>) at kernel/profile.c:143
 dtrace4linux#10 0xffffffff8103a3cf in do_exit ()
 dtrace4linux#11 0xffffffff8103b87c in do_group_exit ()
 dtrace4linux#12 0xffffffff81047522 in get_signal_to_deliver ()
 dtrace4linux#13 0xffffffff81002176 in do_signal ()
 dtrace4linux#14 0xffffffff810025d3 in do_notify_resume ()
 dtrace4linux#15 <signal handler called>
 dtrace4linux#16 0x00007fed3818fff8 in ?? ()

 Refs dtrace4linux#61

driver/taskq.c

@@ -259,6 +259,11 @@ taskqid_t
 taskq_dispatch2(taskq_t *tq, task_func_t func, void *arg, uint_t flags, unsigned long delay)
 {	my_work_t *work;
 
+	if (!tq || !func) {
+		WARN_ON(1);
+		return -1;
+	}
+
 	if (!taskq_enabled)
 		return 0;