Bluetooth: Improve data packing in SAR mode

There is no need to decrease pdu size with L2CAP SDU lenght in Start
L2CAP SDU frame. Start packtet is just 2 bytes longer as specified and
we can keep payload as long as possible.

When testing SAR L2CAP against PTS, L2CAP channel is usually configured
in that way, that SDU = MPS * 3. PTS expets then 3 I-Frames from IUT: Start,
Continuation and End frame.

Without this fix, we sent 4 I-Frames. We could pass a test by using -b
option in l2test and send just two bytes less than SDU length. With this
patch no need to use -b option.

Signed-off-by: Lukasz Rymanowski <lukasz.rymanowski@tieto.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap_core.c

@@ -2275,7 +2275,6 @@ static int l2cap_segment_sdu(struct l2cap_chan *chan,
 	} else {
 		sar = L2CAP_SAR_START;
 		sdu_len = len;
-		pdu_len -= L2CAP_SDULEN_SIZE;
 	}
 
 	while (len > 0) {
@@ -2290,10 +2289,8 @@ static int l2cap_segment_sdu(struct l2cap_chan *chan,
 		__skb_queue_tail(seg_queue, skb);
 
 		len -= pdu_len;
-		if (sdu_len) {
+		if (sdu_len)
 			sdu_len = 0;
-			pdu_len += L2CAP_SDULEN_SIZE;
-		}
 
 		if (len <= pdu_len) {
 			sar = L2CAP_SAR_END;