os/bluestore: strip trailing slash for directory listings

os/bluestore: strip trailing slash for directory listings

Calls to BlueRocksEnv::GetChildren may contain a trailing / in the
queried directory, which is stripped away with this patch.

If it's not stripped, the directory entry is not found in BlueFS:
```
10 bluefs readdir db/
20 bluefs readdir dir db/ not found
 3 rocksdb: [db/db_impl/db_impl_open.cc:1785] Persisting Option File error: OK
```

Fixes: https://tracker.ceph.com/issues/49815
Signed-off-by: Jonas Jelten <jj@sft.lol>
(cherry picked from commit 3aa31813980d22719277a04797df48310acdff66)