extlinux: try to get the disk offset from sysfs

extlinux: try to get the disk offset from sysfs

It is possible(?) that HDIO_GETGEO can't return the full offset, and
in either case it is too small -- only 32 bits on 32-bit platforms.
Thus query sysfs for the real value, if available.

sysfs also contains information for slave devices -- this is going to
really matter for the md/lvm issues.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>