sfc: Support only two rx buffers per page

- Pull the loop handling into efx_init_rx_buffers_(skb|page)
- Remove rx_queue->buf_page, and associated clean up code
- Remove unmap_addr, since unmap_addr is trivially calculable

This will allow us to recycle discarded buffers directly
from efx_rx_packet(), since will never be in the middle of
splitting a page.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>