scsi: scsi_devinfo: cleanly zero-pad devinfo strings

Cleanly fill memory for "vendor" and "model" with 0-bytes for the
"compatible" case rather than adding only a single 0 byte.  This
simplifies the devinfo code a a bit, and avoids mistakes in other places
of the code (not in current upstream, but we had one such mistake in the
SUSE kernel).

[mkp: applied by hand and added braces]

Signed-off-by: Martin Wilck <mwilck@suse.com>
Reviewed-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>