[SCSI] scsi_debug: fix duplicate dif_errors increment

It is unnecessary to increase dif_errors in dif_verify(), because the
caller will increment it when dif_verify() detects failure.

This bug was introduced by commit beb40ea4 ("[SCSI] scsi_debug:
reduce duplication between prot_verify_read and prot_verify_write")

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>