iscsi-target: Fix sendpage breakage with proper padding+DataDigest iovec offsets

This patch fixes a bug in the iscsit_fe_sendpage_sg() transmit codepath that
was originally introduced with the v3.1 iscsi-target merge that incorrectly
uses hardcoded cmd->iov_data_count values to determine cmd->iov_data[] offsets
for extra outgoing padding and DataDigest payload vectors.

This code is obviously incorrect for the DataDigest enabled case with sendpage
offload, and this fix ensures correct operation for padding + DataDigest,
padding only, and DataDigest only cases. The bug was introduced during a
pre-merge change in iscsit_fe_sendpage_sg() to natively use struct scatterlist
instead of the legacy v3.0 struct se_mem logic.

Cc: Andy Grover <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/iscsi/iscsi_target_util.c

···
 	struct kvec iov;
 	u32 tx_hdr_size, data_len;
 	u32 offset = cmd->first_data_sg_off;
-	int tx_sent;
+	int tx_sent, iov_off;
 
 send_hdr:
 	tx_hdr_size = ISCSI_HDR_LEN;
···
 	}
 
 	data_len = cmd->tx_size - tx_hdr_size - cmd->padding;
-	if (conn->conn_ops->DataDigest)
+	/*
+	 * Set iov_off used by padding and data digest tx_data() calls below
+	 * in order to determine proper offset into cmd->iov_data[]
+	 */
+	if (conn->conn_ops->DataDigest) {
 		data_len -= ISCSI_CRC_LEN;
-
+		if (cmd->padding)
+			iov_off = (cmd->iov_data_count - 2);
+		else
+			iov_off = (cmd->iov_data_count - 1);
+	} else {
+		iov_off = (cmd->iov_data_count - 1);
+	}
 	/*
 	 * Perform sendpage() for each page in the scatterlist
 	 */
···
 
 send_padding:
 	if (cmd->padding) {
-		struct kvec *iov_p =
-			&cmd->iov_data[cmd->iov_data_count-1];
+		struct kvec *iov_p = &cmd->iov_data[iov_off++];
 
 		tx_sent = tx_data(conn, iov_p, 1, cmd->padding);
 		if (cmd->padding != tx_sent) {
···
 
 send_datacrc:
 	if (conn->conn_ops->DataDigest) {
-		struct kvec *iov_d =
-			&cmd->iov_data[cmd->iov_data_count];
+		struct kvec *iov_d = &cmd->iov_data[iov_off];
 
 		tx_sent = tx_data(conn, iov_d, 1, ISCSI_CRC_LEN);
 		if (ISCSI_CRC_LEN != tx_sent) {