sfc: Recycle discarded rx buffers back onto the queue

The cut-through design of the receive path means that packets that
fail to match the appropriate MAC filter are not discarded at the MAC
but are flagged in the completion event as 'to be discarded'.  On
networks with heavy multicast traffic, this can account for a
significant proportion of received packets, so it is worthwhile to
recycle the buffer immediately in this case rather than freeing it
and then reallocating it shortly after.

The only complication here is dealing with a page shared
between two receive buffers. In that case, we need to be
careful to free the dma mapping when both buffers have
been free'd by the kernel. This means that we can only
recycle such a page if both receive buffers are discarded.
Unfortunately, in an environment with 1500mtu,
rx_alloc_method=PAGE, and a mixture of discarded and
not-discarded frames hitting the same receive queue,
buffer recycling won't always be possible.

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