Remove HDIO_GETGEO_BIG which is dead (removed from 2.6) and was pointless back

Remove HDIO_GETGEO_BIG which is dead (removed from 2.6) and was pointless back
in 2.4 (it fetched the same hd_geometry struct only with uint cylinders instead
of ushort cylinders: nothing that big has ever accurately reported gemoetry,
it's all LBA.

Also stop declaring the hd_geometry struct static: it's two chars, a short, and
a long for a grand total of 8 bytes.  That can go on the stack.