scsi: set fmt to NULL scsi_extd_sense_format() by default

One of the two callers passes an unintialized pointer as "fmt" and
expects it to be set to NULL if there is no format string.  Let's make
this function work as expected.

Fixes: d811b848

 ('scsi: use sdev as argument for sense code printing')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: James Bottomley <JBottomley@Parallels.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>