Bluetooth: Only check SAR bits if frame is an I-frame

The SAR bits doesn't make sense for an S-frame. It doesn't use SAR.

Checking SAR for a S-frames can lead to L2CAP errors, it could close
the channel with an invalid packet length, since we was removing the 2
of the of any frame that match SAR start bits, without check if it is
an I-frame.
Signed-off-by: NGustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/l2cap.c

@@ -4117,7 +4117,7 @@ static inline int l2cap_data_channel(struct l2cap_conn *conn, u16 cid, struct sk
 		skb_pull(skb, 2);
 		len = skb->len;
 
-		if (__is_sar_start(control))
+		if (__is_sar_start(control) && __is_iframe(control))
 			len -= 2;
 
 		if (pi->fcs == L2CAP_FCS_CRC16)