sfc: convert to SKB paged frag API.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Solarflare linux maintainers <linux-net-drivers@solarflare.com>
Cc: Steve Hodgson <shodgson@solarflare.com>
Cc: Ben Hutchings <bhutchings@solarflare.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>