IBiser: Fix wrong mask when sizeof (dma_addr_t) > sizeof (unsigned long)

The code that prepares the SG associated with SCSI command for FMR was
buggy for systems with DMA addresses that don't fit in unsigned long,
e.g under the 32-bit based XenServer dom0 sizeof(dma_addr_t) is 8.

Fix that by casting to unsigned long long a masking constant used by
the code. This resolves a crash in iser_sg_to_page_vec on this system.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>