loader: geli_dev_ioctl does return huge mediasize

The DIOCGMEDIASIZE is calculated md->md_sectorsize * md->md_provsize, and
for boot disk, the md_sectorsize is 4k. However, the md_provsize is already
in units of bytes.

(cherry picked from commit 0d1a62068123df267e20530c86d2fc7a4e1aaa2e)