mgr/volumes: assert self.fs

mgr/volumes: assert self.fs

fixes mypy errors:

volumes/fs/operations/volume.py: note: In member "disconnect" of class "Connection":
volumes/fs/operations/volume.py:94: error: Item "None" of "Optional[Any]" has no attribute "get_addrs"
volumes/fs/operations/volume.py:95: error: Item "None" of "Optional[Any]" has no attribute "shutdown"

volumes/fs/operations/volume.py: note: In member "abort" of class "Connection":
volumes/fs/operations/volume.py:105: error: Item "None" of "Optional[Any]" has no attribute "abort_conn"

Fixes: https://tracker.ceph.com/issues/44393
Signed-off-by: Michael Fritch <mfritch@suse.com>