Only set blsdir if /boot/loader/entries is in a btrfs or zfs partition
    
    Commit bfc756f8d86 ("Set blsdir if the BLS directory path isn't one of the
    looked up by default") attempted to set blsdir if /boot/loader/entries was
    not the real path of the directory containing the BLS snippets. Which may
    be the case if for example /boot/loader/entries is in a btrfs subvolume.
    
    But in the case of ostree, /boot/loader is a symlink to the directory with
    the entries for the current deployment. So with ostree the blsdir will be
    wrongly set, since GRUB is able to follow the symlinks just fine. In fact,
    it has to follow the symlink since otherwise GRUB will always use the BLS
    files for the deployment that the symlink pointed out when blsdir was set.
    
    So only set blsdir if /boot/loader/entries is in a btrfs or zfs partition.
    
    Related: rhbz#1688453
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>