VMW_PVSCSI: Fix the issue of DMA-API related warnings.

The driver is missing calls to pci_dma_mapping_error() after
performing the DMA mapping, which caused DMA-API warning to
show up in dmesg's output. Though that happens only when
DMA_API_DEBUG option is enabled. This change fixes the issue
and makes pvscsi_map_buffers() function more robust.

Signed-off-by: Arvind Kumar <arvindkumar@vmware.com>
Cc: Josh Boyer <jwboyer@fedoraproject.org>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Josh Boyer <jwboyer@fedoraproject.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>