mgr/nfs/utils: changes to helper func to check cephfs path

mgr/nfs/utils: changes to helper func to check cephfs path

- Renamed to cephfs_path_is_dir

- Removed exception handling to prevent redundant log statements like:
   "No such file or directory error in stat: b'/mnt/testdir_symlink': No such file or directory [Errno 2]"

  Exceptions handled inside caller eliminates this redundancy

- Set modifier flag AT_SYMLINK_NOFOLLOW

- Removed string "{path} is not a dir" when raising NotADirectoryError
  Rationale: will be handled in export.py

- change mock to cephfs_path_is_dir

Signed-off-by: Dhairya Parmar <dparmar@redhat.com>