report: lv_permissions: display read-only/read-only-override instead of blank when...

report: lv_permissions: display read-only/read-only-override instead of blank when LV is not writeable

Before this patch:
$ lvs -o name,lv_attr,lv_permissions
  LV    Attr       LPerms
  root  -wi-ao---- writeable
  swap  -wi-ao---- writeable
  lvol0 -Ri-a-----
  lvol1 -ri-a-----
  lvol2 -wi-a----- writeable
  lvol3 -ri-------

With this patch applied:
$ lvs -o name,lv_attr,lv_permissions
  LV    Attr       LPerms
  root  -wi-ao---- writeable
  swap  -wi-ao---- writeable
  lvol0 -Ri-a----- read-only-override
  lvol1 -ri-a----- read-only
  lvol2 -wi-a----- writeable
  lvol3 -ri------- read-only

("read-only-override" is displayed if the "writeable" permission
is overriden based on "activation/read_only_volume_list" lvm.conf
setting)