[CIFS] Fix spurious reconnect on 2nd peek from read of SMB length

When retrying kernel_recvmsg() because of a short read, check returned
length against the remaining length, not against total length. This
avoids unneeded session reconnects which would otherwise occur when
kernel_recvmsg() finally returns zero when asked to read zero bytes.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/connect.c

@@ -438,9 +438,9 @@ cifs_demultiplex_thread(struct TCP_Server_Info *server)
 			csocket = server->ssocket;
 			wake_up(&server->response_q);
 			continue;
-		} else if (length < 4) {
-			cFYI(1, ("less than four bytes received (%d bytes)",
-			      length));
+		} else if (length < pdu_length) {
+			cFYI(1, ("requested %d bytes but only got %d bytes",
+				  pdu_length, length));
 			pdu_length -= length;
 			msleep(1);
 			goto incomplete_rcv;