[SCSI] scsi_debug: fix incorrectly nested kmap_atomic()

In the function prot_verify_write(), kmap_atomic()/kunmap_atomic() for
data page and kmap_atomic()/kunmap_atomic() for protection information
page are not nested each other.

It worked perfectly before commit 3e4d3af5
("mm: stack based kmap_atomic()").  Because the kmap_atomic slot KM_IRQ0
was used for data page and the slot KM_IRQ1 was used for protection page.

But KM_types are gone and kmap_atomic() is using stack based implementation.
So two different kmap_atomic() usages must be strictly nested now.

This change ensures kmap_atomic() usage is strictly nested.

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>