Increase maximum receive ring size from 255 to 1020 by supporting
up to 4 linked pages of receive descriptors. To accomodate the
higher memory usage, each physical descriptor page is allocated
separately and the software ring that keeps track of the SKBs and the
DMA addresses is allocated using vmalloc.

Some of the receive-related fields in the bp structure are re-
organized a bit for better locality of reference.

The max. was reduced to 1020 from 4080 after discussion with David
Miller.

This patch contains ring init code changes only. This next patch
contains rx data path code changes.
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>