[BNX2]: Enable S/G for jumbo RX.

If the MTU requires more than 1 page for the SKB, enable the page ring
and calculate the size of the page ring.  This will guarantee order-0
allocation regardless of the MTU size.

Fixup loopback test packet size so that we don't deal with the pages
during loopback test.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>