smb: client: handle lack of IPC in dfs_cache_refresh()

In very rare cases, DFS mounts could end up with SMB sessions without
any IPC connections.  These mounts are only possible when having
unexpired cached DFS referrals, hence not requiring any IPC
connections during the mount process.

Try to establish those missing IPC connections when refreshing DFS
referrals.  If the server is still rejecting it, then simply ignore
and leave expired cached DFS referral for any potential DFS failovers.

Reported-by: Jay Shin <jaeshin@redhat.com>
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>