loader: zfs reader should not probe partitionless disks
See also: https://reviews.freebsd.org/D18558
First of all, normal setups can not boot such pools as the tools
do not support installing boot programs.
Secondly, for proper pool configuration detection, we need to checks all
four label copies on disk, 2 from front and 2 from the end of the disk,
but zfs label does not contain the size of the disk - so we depend on
firmware to report the correct disk size or use information from the
Without partition table, we only can rely on firmware to report and support
disk IO properly.
There is a specific case: 8TB disks are reported by BIOS to have 4294967295
sectors (0x00000000ffffffff), the sectors reported by OS is 15628053168
(0x00000003a3812ab0), so the reported size is less than actual but is hitting
32-bit max. Unfortuantely the real limit must be even lower because probing
this disk in this system will wnd up with hung system.
UEFI boot of this system seems not to be affected.