Fix possible pool hold leak in dmu_send_impl()

Problem reported to vendor:
  https://www.illumos.org/issues/3645

Reported by:	Andriy Gapon <avg@FreeBSD.org>
MFC after:	15 days

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_send.c

@@ -500,14 +500,14 @@ dmu_send_impl(void *tag, dsl_pool_t *dp, dsl_dataset_t *ds,
 	list_insert_head(&ds->ds_sendstreams, dsp);
 	mutex_exit(&ds->ds_sendstream_lock);
 
+	dsl_dataset_long_hold(ds, FTAG);
+	dsl_pool_rele(dp, tag);
+
 	if (dump_bytes(dsp, drr, sizeof (dmu_replay_record_t)) != 0) {
 		err = dsp->dsa_err;
 		goto out;
 	}
 
-	dsl_dataset_long_hold(ds, FTAG);
-	dsl_pool_rele(dp, tag);
-
 	err = traverse_dataset(ds, fromtxg, TRAVERSE_PRE | TRAVERSE_PREFETCH,
 	    backup_cb, dsp);