smb: client: fix DFS interlink failover

The DFS interlinks point to different DFS namespaces so make sure to
use the correct DFS root server to chase any DFS links under it by
storing the SMB session in dfs_ref_walk structure and then using it on
every referral walk.

Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>