[SCSI] scsi_debug: reduce duplication between prot_verify_read and prot_verify_write

In order to reduce code duplication between prot_verify_read() and
prot_verify_write(), this moves common code into the new functions.

[jejb: fix unitialised variable warning]
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Acked-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>