cifs: set a minimum of 2 minutes for refreshing dfs cache

We don't want to refresh the dfs cache in very short intervals, so
setting a minimum interval of 2 minutes is OK.

If it needs to be refreshed immediately, one could have the cache
cleared with

	$ echo 0 > /proc/fs/cifs/dfscache

and then remounting the dfs share.

Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/dfs_cache.c

@@ -24,6 +24,7 @@
 
 #define CACHE_HTABLE_SIZE 32
 #define CACHE_MAX_ENTRIES 64
+#define CACHE_MIN_TTL 120 /* 2 minutes */
 
 #define IS_INTERLINK_SET(v) ((v) & (DFSREF_REFERRAL_SERVER | \
 				    DFSREF_STORAGE_SERVER))
@@ -492,7 +493,7 @@ static int copy_ref_data(const struct dfs_info3_param *refs, int numrefs,
 {
 	int i;
 
-	ce->ttl = refs[0].ttl;
+	ce->ttl = max_t(int, refs[0].ttl, CACHE_MIN_TTL);
 	ce->etime = get_expire_time(ce->ttl);
 	ce->srvtype = refs[0].server_type;
 	ce->hdr_flags = refs[0].flags;