libceph: move debugfs initialization into __ceph_open_session()

Our debugfs dir name is a concatenation of cluster fsid and client
unique ID ("global_id").  It used to be the case that we learned
global_id first, nowadays we always learn fsid first - the monmap is
sent before any auth replies are.  ceph_debugfs_client_init() call in
ceph_monc_handle_map() is therefore never executed and can be removed.

Its counterpart in handle_auth_reply() doesn't really belong there
either: having to do monc->client and unlocking early to work around
lockdep is a testament to that.  Move it into __ceph_open_session(),
where it can be called unconditionally.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>