iscsi-target: Fail connection on short sendmsg writes

This patch changes iscsit_do_tx_data() to fail on short writes
when kernel_sendmsg() returns a value different than requested
transfer length, returning -EPIPE and thus causing a connection
reset to occur.

This avoids a potential bug in the original code where a short
write would result in kernel_sendmsg() being called again with
the original iovec base + length.

In practice this has not been an issue because iscsit_do_tx_data()
is only used for transferring 48 byte headers + 4 byte digests,
along with seldom used control payloads from NOPIN + TEXT_RSP +
REJECT with less than 32k of data.

So following Al's audit of iovec consumers, go ahead and fail
the connection on short writes for now, and remove the bogus
logic ahead of his proper upstream fix.
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: David S. Miller <davem@davemloft.net>
Cc: <stable@vger.kernel.org> # v3.1+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_util.c

@@ -1358,15 +1358,15 @@ static int iscsit_do_tx_data(
 	struct iscsi_conn *conn,
 	struct iscsi_data_count *count)
 {
-	int data = count->data_length, total_tx = 0, tx_loop = 0, iov_len;
+	int ret, iov_len;
 	struct kvec *iov_p;
 	struct msghdr msg;
 
 	if (!conn || !conn->sock || !conn->conn_ops)
 		return -1;
 
-	if (data <= 0) {
-		pr_err("Data length is: %d\n", data);
+	if (count->data_length <= 0) {
+		pr_err("Data length is: %d\n", count->data_length);
 		return -1;
 	}
 
@@ -1375,20 +1375,16 @@ static int iscsit_do_tx_data(
 	iov_p = count->iov;
 	iov_len = count->iov_count;
 
-	while (total_tx < data) {
-		tx_loop = kernel_sendmsg(conn->sock, &msg, iov_p, iov_len,
-					(data - total_tx));
-		if (tx_loop <= 0) {
-			pr_debug("tx_loop: %d total_tx %d\n",
-				tx_loop, total_tx);
-			return tx_loop;
-		}
-		total_tx += tx_loop;
-		pr_debug("tx_loop: %d, total_tx: %d, data: %d\n",
-					tx_loop, total_tx, data);
+	ret = kernel_sendmsg(conn->sock, &msg, iov_p, iov_len,
+			     count->data_length);
+	if (ret != count->data_length) {
+		pr_err("Unexpected ret: %d send data %d\n",
+		       ret, count->data_length);
+		return -EPIPE;
 	}
+	pr_debug("ret: %d, sent data: %d\n", ret, count->data_length);
 
-	return total_tx;
+	return ret;
 }
 
 int rx_data(