[SCSI] scsi_debug: factor out copying PI from dif_storep to prot_sglist

If data integrity support is enabled, prot_verify_read() is called in
response to READ commands and it verifies protection info from dif_storep
by comparing against fake_storep, and copies protection info to
prot_sglist.

This factors out the portion of copying protection info into a separate
function.  It will also be reused in the next change after supporting
the opposite direction (copying prot_sglist to dif_storep).

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