scsi: sd: improve TUR handling in sd_check_events

Remove bogus evaluations of retval and sshdr when the device is offline,
and fix a possible NULL pointer dereference by allocating the 8 byte
sized sense header on stack.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>