Bluetooth: l2cap: Check encryption key size on incoming connection

This is required for passing GAP/SEC/SEM/BI-04-C PTS test case:
  Security Mode 4 Level 4, Responder - Invalid Encryption Key Size
  - 128 bit

This tests the security key with size from 1 to 15 bytes while the
Security Mode 4 Level 4 requests 16 bytes key size.

Currently PTS fails with the following logs:
- expected:Connection Response:
    Code: [3 (0x03)] Code
    Identifier: (lt)WildCard: Exists(gt)
    Length: [8 (0x0008)]
    Destination CID: (lt)WildCard: Exists(gt)
    Source CID: [64 (0x0040)]
    Result: [3 (0x0003)] Connection refused - Security block
    Status: (lt)WildCard: Exists(gt),
but received:Connection Response:
    Code: [3 (0x03)] Code
    Identifier: [1 (0x01)]
    Length: [8 (0x0008)]
    Destination CID: [64 (0x0040)]
    Source CID: [64 (0x0040)]
    Result: [0 (0x0000)] Connection Successful
    Status: [0 (0x0000)] No further information available

And HCI logs:
< HCI Command: Read Encrypti.. (0x05|0x0008) plen 2
        Handle: 14 Address: 00:1B:DC:F2:24:10 (Vencer Co., Ltd.)
> HCI Event: Command Complete (0x0e) plen 7
      Read Encryption Key Size (0x05|0x0008) ncmd 1
        Status: Success (0x00)
        Handle: 14 Address: 00:1B:DC:F2:24:10 (Vencer Co., Ltd.)
        Key size: 7
> ACL Data RX: Handle 14 flags 0x02 dlen 12
      L2CAP: Connection Request (0x02) ident 1 len 4
        PSM: 4097 (0x1001)
        Source CID: 64
< ACL Data TX: Handle 14 flags 0x00 dlen 16
      L2CAP: Connection Response (0x03) ident 1 len 8
        Destination CID: 64
        Source CID: 64
        Result: Connection successful (0x0000)
        Status: No further information available (0x0000)

Fixes: 288c069 ("Bluetooth: Enforce key size of 16 bytes on FIPS level")
Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/l2cap_core.c

@@ -3991,7 +3991,8 @@ static void l2cap_connect(struct l2cap_conn *conn, struct l2cap_cmd_hdr *cmd,
 
 	/* Check if the ACL is secure enough (if not SDP) */
 	if (psm != cpu_to_le16(L2CAP_PSM_SDP) &&
-	    !hci_conn_check_link_mode(conn->hcon)) {
+	    (!hci_conn_check_link_mode(conn->hcon) ||
+	    !l2cap_check_enc_key_size(conn->hcon))) {
 		conn->disc_reason = HCI_ERROR_AUTH_FAILURE;
 		result = L2CAP_CR_SEC_BLOCK;
 		goto response;