mgr/dashboard: fix error when typing existing paths in the Ganesha form

mgr/dashboard: fix error when typing existing paths in the Ganesha form

- The `CephFS.ls_dir()` implementation in the backend had changed, the code in the
  UI endpoint `/ui-api/nfs-ganesha/lsdir` needs to adapt.
- Add fs_name as resource_id in `/ui-api/nfs-ganesha/lsdir/<fs_name>` to distinguish FS.
- Add more checks and unit tests.

Fixes: https://tracker.ceph.com/issues/47372
Signed-off-by: Kiefer Chang <kiefer.chang@suse.com>
(cherry picked from commit ee14ef7c6095d0572e90b29580f2d8d6982dcbda)

Conflicts:
	src/pybind/mgr/dashboard/controllers/nfsganesha.py
	src/pybind/mgr/dashboard/tests/test_ganesha.py
        - Conflict due to import order changed in the master