cifs: Fix DFS cache refresher for DFS links

As per MS-DFSC, when a DFS cache entry is expired and it is a DFS
link, then a new DFS referral must be sent to root server in order to
refresh the expired entry.

This patch ensures that all new DFS referrals for refreshing the cache
are sent to DFS root.

Signed-off-by: Paulo Alcantara (SUSE) <paulo@paulo.ac>
Signed-off-by: Steve French <stfrench@microsoft.com>