Bluetooth: SCO: Fix UAF on sco_sock_timeout

conn->sk maybe have been unlinked/freed while waiting for sco_conn_lock
so this checks if the conn->sk is still valid by checking if it part of
sco_sk_list.

Reported-by:  <syzbot+4c0d0c4cde787116d465@syzkaller.appspotmail.com>
Tested-by:  <syzbot+4c0d0c4cde787116d465@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=4c0d0c4cde787116d465


Fixes: ba316be1 ("Bluetooth: schedule SCO timeouts with delayed_work")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>