Issue openzfs#4116.

Make zvol update volsize operation synchronous.

There is a race condition when new transaction group is added
to dp->dp_dirty_datasets list by the zap_update in the zvol_update_volsize.
Meanwhile, before these dirty data are synchronized, the receive process
can cause that dmu_recv_end_sync is executed. Then finally dirty data
are going to be synchronized but the synchronization ends with the NULL
pointer dereference error.

module/zfs/zvol.c

@@ -276,10 +276,12 @@ zvol_update_volsize(uint64_t volsize, objset_t *os)
 {
 	dmu_tx_t *tx;
 	int error;
+	uint64_t txg;
 
 	ASSERT(MUTEX_HELD(&zvol_state_lock));
 
 	tx = dmu_tx_create(os);
+	txg = dmu_tx_get_txg(tx);
 	dmu_tx_hold_zap(tx, ZVOL_ZAP_OBJ, TRUE, NULL);
 	dmu_tx_mark_netfree(tx);
 	error = dmu_tx_assign(tx, TXG_WAIT);
@@ -292,6 +294,8 @@ zvol_update_volsize(uint64_t volsize, objset_t *os)
 	    &volsize, tx);
 	dmu_tx_commit(tx);
 
+	txg_wait_synced(dmu_objset_pool(os), txg);
+
 	if (error == 0)
 		error = dmu_free_long_range(os,
 		    ZVOL_OBJ, volsize, DMU_OBJECT_END);