loader: uboot disk ioctl should call disk_ioctl

As the uboot disk interface is using common/disk.c API, we also
should use disk_ioctl() call, this will give us chance to read partition
sizes and have feature parity with UEFI and BIOS implementations.

This does also fix arm boot issue on some systems, reported/tested by Ian,
thanks.

Reported by: ian
Reviewed by: ian
Differential Revision: https://reviews.freebsd.org/D10421