stand/zfs: Add a third argument to zfs_probe_dev: part_too

Pass in 'true' if you'd like to search this device's partitions or
'false' if you should just search the device. EFI and (in the future)
kboot have discrete partitions that aren't accessed via the full disk
device. Weird things happen if you try to search in these cases.

Sponsored by: Netflix

(cherry picked from commit 71bbe6fb709dcfbe36877fa53c928e2e6d881843)