loader: dosfs fails to access directory data with large cluster number

fsize is using 2 bytes for cluster number, but with fat32 we
actually do have 4 bytes and with large disks the high bytes will be in use.

illumos issue: https://www.illumos.org/issues/16821

Sponsored by: MNX Cloud, Inc.

(cherry-picked from commit 79a0d14fa05c75038caa52099289effb85732f88)