tools/ceph_objectstore_tool: action_on_all_objects_in_pg to skip pgmeta

tools/ceph_objectstore_tool: action_on_all_objects_in_pg to skip pgmeta

See "Error getting attr on" after creating an empty pool:

```
$ ceph-objectstore-tool --data-path ./dev/osd0 --op list
Error getting attr on : 2.0_head,#2:00000000::::head#, (61) No data available
["2.0",{"oid":"","key":"","snapid":-2,"hash":0,"max":0,"pool":2,"namespace":"","max":0}]
```

_action_on_all_objects_in_pg() should skip getting the (non-existing) attributes of pgmeta hobjects.

Note: pgmeta_oid is created at the PG's constructor and is written to disk in init_pg_ondisk()
      without any OI_ATTR.

Fixes: https://tracker.ceph.com/issues/63640
Signed-off-by: Matan Breizman <mbreizma@redhat.com>