[SCSI] scsi_debug: fix duplicate dif_errors increment

[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 beb40ea42bd6 ("[SCSI] scsi_debug:
reduce duplication between prot_verify_read and prot_verify_write")

Signed-off-by: Akinobu Mita <[email protected]>
Acked-by: Martin K. Petersen <[email protected]>
Reviewed-by: Douglas Gilbert <[email protected]>
Signed-off-by: James Bottomley <[email protected]>