Return call to init_zfs_bootenv to its previous location

When called to early, new_currdev->d_type was not yet set
zfs_fmtdev() would then return null

While here, guard call to init_zfs_bootenv with if d_type == DEVT_ZFS

Reported by: tsoome at me.com
MFC after: 3 days
Sponsored by: ScaleEngine Inc.