We can't acquire volume lock while refreshing the DFS cache because
cifs_reconnect() may call dfs_cache_update_vol() while we are walking
through the volume list.

To prevent that, make vol_info refcounted, create a temp list with all
volumes eligible for refreshing, and then use it without any locks
held.

Besides, replace vol_lock with a spinlock and protect cache_ttl from
concurrent accesses or changes.
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>