cifs: fix refresh of cached referrals

We can't rely on cifs_tcon::ses to refresh cached referral as the
server target might not respond to referrals, e.g. share is not hosted
in a DFS root server.  Consider the following

  mount //dom/dfs/link -> /root1/dfs/link -> /fs0/share

where fs0 can't get a referral for "/root1/dfs/link".

To simplify and fix the access of dfs root sessions, store the dfs
root session pointer directly to new sessions so making it easier to
select the appropriate ipc connection and use it for failover or cache
refresh.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>