sata_mv: Warn about HPT RocketRAID BIOS treatment of "Legacy" drives

The Highpoint RocketRAID boards using Marvell 7042 chips
overwrite the 9th sector of attached drives at boot time,
when those drives are configured as "Legacy" (the default)
in the HighPoint BIOS.

This kills GRUB, and probably other stuff.
But it all happens *before* Linux is even loaded.

So, for now we'll log a WARNING when such boards are detected,
and advise users to configure BIOS "JBOD" volumes instead,
which don't appear to suffer from this problem.

Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>