loader: reading from large dosfs fails

With 8GB disk image and FAT32, our read offset calculation wraps over
32-bit integer and we end up reading garbage. The problem appears when
disk image is filled with data and the block to bytes translations do
not fit into 32-bit integers.

illumos issue: https://www.illumos.org/issues/16666

Sponsored by: MNX Cloud, Inc.
MFC after: 1 week