smb: client: fix DFS failover in multiuser mounts

For sessions and tcons created on behalf of new users accessing a
multiuser mount, matching their sessions in tcon_super_cb() with
master tcon will always lead to false as every new user will have its
own session and tcon.

All multiuser sessions, however, will inherit ->dfs_root_ses from
master tcon, so match it instead.

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