mlx4: fix DMA mapping leak when allocation fails

mlx4_en_prepare_rx_desc does not correctly clean up after it finds an
allocation failure. It should unmap a page before calling put_page, but
it only calls the later.

This bug would prevent a device removal using hotplug after setting the
device MTU to 9000 and opening the network interface. After the fix, we
still see the allocation failure with MTU 9000, but we are able to
remove the device.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>