moving zgenhostid up - before dracut (openzfs#566)

we need to generate hostid before creating initramfs image, otherwise, you may receive the following warning when importing your pool as root:
```
$ sudo zpool status
  pool: rpool
 state: ONLINE
status: Mismatch between pool hostid and system hostid on imported pool.
	This pool was previously imported into a system with a different hostid,
	and then was verbatim imported into this system.
action: Export this pool on all systems on which it is imported.
	Then import it to correct the mismatch.
   see: https://openzfs.github.io/openzfs-docs/msg/ZFS-8000-EY
config:

	NAME        STATE     READ WRITE CKSUM
	rpool       ONLINE       0     0     0
	  sda3      ONLINE       0     0     0

errors: No known data errors
```

Author: akaramimotlagh