If the module parameter virtual_gb is greater than 0, the READ command
may request the blocks which exceed actual ramdisk storage (fake_storep).
prot_verify_read() should treat those blocks as wrap around the end of
fake_storep.  But it actually causes fake_storep and dif_storep buffer
overruns.

This fixes these buffer overruns.  In order to simplify the fix,
this also introduces fake_store() and dif_store() which return
corresponding wrap around addresses.
Signed-off-by: NAkinobu Mita <akinobu.mita@gmail.com>
Acked-by: NMartin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: NJames Bottomley <JBottomley@Parallels.com>