Bluetooth: Set P-bit for SREJ frame only if there are I-frames to ack

SREJ frame with P-bit set acknowledges I-frames numbered up to
(ReqSeq - 1). With this patch P-bit in SREJ is set only when there are
some I-frames to ack.

This fixes ambiguous situation when lost of I-frame with TxSeq=0 would
result in sending SREJ acking all previous I-frames.
Consider following scenario:
TxWindow=3

HostA: sent I-frame TxSeq=0
HostA: sent I-frame TxSeq=1
HostA: sent I-frame TxSeq=2
HostB: missed I-frame TxSeq=0
HostB: received I-frame TxSeq=1
HostB: sent SREJ ReqSeq=0 Pbit=1
HostA: received SREJ ReqSeq=0 Pbit=1   <- All I-frames acked or not?
...
Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>

net/bluetooth/l2cap_core.c

@@ -3927,15 +3927,15 @@ static inline int l2cap_data_channel_iframe(struct l2cap_chan *chan, u32 rx_cont
 		__skb_queue_head_init(&chan->srej_q);
 		l2cap_add_to_srej_queue(chan, skb, tx_seq, sar);
 
-		set_bit(CONN_SEND_PBIT, &chan->conn_state);
+		/* Set P-bit only if there are some I-frames to ack. */
+		if (__clear_ack_timer(chan))
+			set_bit(CONN_SEND_PBIT, &chan->conn_state);
 
 		err = l2cap_send_srejframe(chan, tx_seq);
 		if (err < 0) {
 			l2cap_send_disconn_req(chan->conn, chan, -err);
 			return err;
 		}
-
-		__clear_ack_timer(chan);
 	}
 	return 0;