loader: simplify efi_zfs_probe and avoid double probing for zfs.

[tsoome]

Summary

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 this with
pointer for pool GUID.

[allanjude]

Does this change which pool might be booted from if you have more than 1 pool?

How have you tested this change?

[tsoome]

In D10198#210891, @allanjude wrote:

Does this change which pool might be booted from if you have more than 1 pool?

How have you tested this change?

The first found pool from the boot disk will be booted - I mean, we still do use the device handle from loaded image and use it to read out the boot pool GUID, just using much less complicated way. Testing is the usual - build/boot; also the same implementation is used in my illumos loader port and ok, I do not have multiple pools on the same disk, but I do have multiple pools on different disks in my test VM. So far i haven't seen anything unexpected.

[allanjude]

Approved By: allanjude