exec_drive_taskfile() checks for dma mapping errors by comparison
returned address with zero, while pci_dma_mapping_error() should be used.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: NAlexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: NJens Axboe <axboe@fb.com>