cifs: do not share tcons with DFS

[ Upstream commit 65303de8 ]

This disables tcon re-use for DFS shares.

tcon->dfs_path stores the path that the tcon should connect to when
doing failing over.

If that tcon is used multiple times e.g. 2 mounts using it with
different prefixpath, each will need a different dfs_path but there is
only one tcon. The other solution would be to split the tcon in 2
tcons during failover but that is much harder.

tcons could not be shared with DFS in cifs.ko because in a
DFS namespace like:

          //domain/dfsroot -> /serverA/dfsroot, /serverB/dfsroot

          //serverA/dfsroot/link -> /serverA/target1/aa/bb

          //serverA/dfsroot/link2 -> /serverA/target1/cc/dd

you can see that link and link2 are two DFS links that both resolve to
the same target share (/serverA/target1), so cifs.ko will only contain a
single tcon for both link and link2.

The problem with that is, if we (auto)mount "link" and "link2", cifs.ko
will only contain a si...