init_ring(), refill_rx_ring() and start_tx() don't check
if mapping dma memory succeed.
The patch adds the checks and failure handling.

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>