Fix gcc cast warnings

Gcc -Wall warn: 'lacks a cast'
Gcc -Wall warn: 'comparison between pointer and integer'

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zil.c

@@ -1299,7 +1299,7 @@ zil_clean(zilog_t *zilog, uint64_t synced_txg)
 	 * created a bad performance problem.
 	 */
 	if (taskq_dispatch(zilog->zl_clean_taskq,
-	    (void (*)(void *))zil_itxg_clean, clean_me, TQ_NOSLEEP) == NULL)
+	    (void (*)(void *))zil_itxg_clean, clean_me, TQ_NOSLEEP) == 0)
 		zil_itxg_clean(clean_me);
 }