[SCSI] Fix printing of failed 32-byte commands

Having the large CDB allocation logic in sd.c means that
scsi_io_completion does not have access to the command buffer. That in
turn causes garbage to be printed when a 32-byte command fails. Move the
command printing to sd_done where the command buffer is intact.  Clear
the command buffer pointer after the extended CDB has been freed.

Make scsi_print_command ignore commands with NULL CDB pointers to
inhibit printing of garbled command strings.

Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>