libata: HPA support

Signed-off-by: Alan Cox <alan@redhat.com>
Add support for ignoring the BIOS HPA result (off by default) and setting
the disk to the full available size unless already frozen.

Tested with various platforms/disks and confirmed to work with the
Macintosh (which broke earlier) and ata_piix (breakage due to the LBA48
readback that Tejun fixed).

For normal users this brings us, I believe, to feature parity with old IDE
(and of course more featured in some areas too).
Signed-off-by: Jeff Garzik <jeff@garzik.org>