Rename EMPTY_TASKQ into taskq_empty

To follow a change in illumos taskq

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #12802

include/os/linux/spl/sys/taskq.h

@@ -163,6 +163,8 @@ extern taskq_t *taskq_of_curthread(void);
 	((void) sizeof (dc), \
 	    taskq_create(name, nthreads, maxclsyspri, min, max, flags))
 
+extern boolean_t taskq_empty(taskq_t *);
+
 int spl_taskq_init(void);
 void spl_taskq_fini(void);