loader: simplify efi_zfs_probe and avoid double probing for zfs.

The current efi_zfs_probe() is overcomplicated and can be made
simpler. Still we need to pick up the device handle for our boot
disk first, because the ESP does not have to be the first partition on the
disk.

Once we do have the handle for boot disk, we probe that disk with
pointer for pool GUID.

Reviewed by: allanjude
Approved by: allanjude (mentor)
Differential Revision: https://reviews.freebsd.org/D10198