put stricter guards on queue dead checks

put stricter guards on queue dead checks

commit 86cbfb5607d4b81b1a993ff689bbd2addd5d3a9b upstream.

SCSI uses request_queue->queuedata == NULL as a signal that the queue
is dying.  We set this state in the sdev release function.  However,
this allows a small window where we release the last reference but
haven't quite got to this stage yet and so something will try to take
a reference in scsi_request_fn and oops.  It's very rare, but we had a
report here, so we're pushing this as a bug fix

The actual fix is to set request_queue->queuedata to NULL in
scsi_remove_device() before we drop the reference.  This causes
correct automatic rejects from scsi_request_fn as people who hold
additional references try to submit work and prevents anything from
getting a new reference to the sdev that way.

Signed-off-by: James Bottomley <James.Bottomley@suse.de>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>