sata_mv: improve warnings about Highpoint RocketRAID 23xx cards

Improve the existing boot/load time warnings from sata_mv
for Highpoint RocketRAID 23xx cards, based on new knowledge
about where the BIOS likes to overwrite sectors with metadata.

Harmless to us, but very useful for end users.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>