Sanitizing a geometry in libdisk

The Sanitize_Bios_Geom() is always called after r188408.  But the
function has a problem of checking a disk size.  So it causes to using
incorrect geometry.

I think that it's the same problem in g_part_bsd_read()_at_g_part_bsd.c

The attached patch is useful for pc98 at least so it should be in 8.0.

Please review it.

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TAKAHASHI Yoshihiro <nyan_at_jp.FreeBSD.org>