sfc: Allow shared pages to be recycled

Insert a structure at the start of the shared page that
tracks the dma mapping refcnt. DMA into the next cache
line of the (shared) page (plus EFX_PAGE_IP_ALIGN).

When recycling a page, check the page refcnt. If the
page is otherwise unused, then resurrect the other
receive buffer that previously referenced the page.
Be careful not to overflow the receive ring, since we
can now resurrect n receive buffers in a row.

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