From: 	Heiko Carstens <Heiko.Carstens@de.ibm.com>

This happened when sending out the inquiry command and the LLDD was in
recovery for the specific LUN and returned SCSI_MLQUEUE_DEVICE_BUSY.
Problem is that max_device_blocked gets set _after_ the inquiry command
finished. In this specific case max_device_blocked was 0 and thus the
device was never blocked resulting in the observed stack overflow.
I moved the initializazion of max_device_blocked from scsi_add_lun to
sdev_alloc_sdev and the problem is gone.