Bluetooth: hci_conn: return ERR_PTR instead of NULL when there is no link

hci_connect_sco currently returns NULL when there is no link (i.e. when
hci_conn_link() returns NULL).

sco_connect() expects an ERR_PTR in case of any error (see line 266 in
sco.c). Thus, hcon set as NULL passes through to sco_conn_add(), which
tries to get hcon->hdev, resulting in dereferencing a NULL pointer as
reported by syzkaller.

The same issue exists for iso_connect_cis() calling hci_connect_cis().

Thus, make hci_connect_sco() and hci_connect_cis() return ERR_PTR
instead of NULL.

Reported-and-tested-by: syzbot+37acd5d80d00d609d233@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=37acd5d80d00d609d233
Fixes: 06149746 ("Bluetooth: hci_conn: Add support for linking multiple hcon")
Signed-off-by: Siddh Raman Pant <code@siddh.me>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

net/bluetooth/hci_conn.c

@@ -1684,7 +1684,7 @@ struct hci_conn *hci_connect_sco(struct hci_dev *hdev, int type, bdaddr_t *dst,
 	if (!link) {
 		hci_conn_drop(acl);
 		hci_conn_drop(sco);
-		return NULL;
+		return ERR_PTR(-ENOLINK);
 	}
 
 	sco->setting = setting;
@@ -2254,7 +2254,7 @@ struct hci_conn *hci_connect_cis(struct hci_dev *hdev, bdaddr_t *dst,
 	if (!link) {
 		hci_conn_drop(le);
 		hci_conn_drop(cis);
-		return NULL;
+		return ERR_PTR(-ENOLINK);
 	}
 
 	/* If LE is already connected and CIS handle is already set proceed to