mkfs: fix filesystem and log size units

The actual thresholds enforced by mkfs are in MiB and GiB but the
error messages and documentation report the limits as MB and GB.

Update the filesystem and log size threshold messages, comments,
and man page to use MiB and GiB.

Signed-off-by: Manognya Singuru <msinguru@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>