zfs/sparc64 boot: fix booting after r242230
arch_zfs_probe method is supposed to only probe for ZFS vdevs, but it can
not expect that ZFS data is in a ready state yet.
So, move some code from sparc64_zfs_probe to main to meet the constraints.
Reported by: Chris Ross <cross+freebsd@distal.com>
Tested by: Chris Ross <cross+freebsd@distal.com>
MFC after: 4 days