Fix kernel oops in l2cap_chan_destroy. 

3.6.11 oopses under certain situations when using bluetooth - see http://www.raspberrypi.org/phpBB3/viewtopic.php?f=28&t=37265

I found a fix for this in the mainline kernel source and backported it to rpi-3.6.y

Based on https://github.com/torvalds/linux/commit/4af66c691f4e5c2db9bb00793669a548e9db1974