smb: client: don't retry DFS targets on server shutdown

If TCP Server is about to be destroyed (e.g. CifsExiting was set) and
it is reconnecting, stop retrying DFS targets from cached DFS referral
as this would potentially delay server shutdown in several seconds.

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