[SCSI] sd: fix bug in SCSI async probing

The async split up of probing in sd.c created a potential failure case where
something goes wrong with device_add(), but which we don't recover properly.
Since, in general, asynchronous error handling is hard, move the device_add()
into the asynchronous path (it should be fast) and make sure all the deferred
processing cannot fail.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>