be2iscsi: add checks for dma mapping errors

hwi_write_buffer() does not check if mapping dma memory succeed. The
patch adds the check and failure handling.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Reviewed-by: Jitendra Bhivare <jitendra.bhivare@avagotech.com>
Acked-by: Jitendra Bhivare <jitendra.bhivare@avagotech.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>