fix prune_dcache()/umount() race

... and get rid of the last __put_super_and_need_restart() caller
Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -536,7 +536,7 @@ static void __shrink_dcache_sb(struct super_block *sb, int *count, int flags)
  */
 static void prune_dcache(int count)
 {
-	struct super_block *sb;
+	struct super_block *sb, *n;
 	int w_count;
 	int unused = dentry_stat.nr_unused;
 	int prune_ratio;
@@ -545,13 +545,12 @@ static void prune_dcache(int count)
 	if (unused == 0 || count == 0)
 		return;
 	spin_lock(&dcache_lock);
-restart:
 	if (count >= unused)
 		prune_ratio = 1;
 	else
 		prune_ratio = unused / count;
 	spin_lock(&sb_lock);
-	list_for_each_entry(sb, &super_blocks, s_list) {
+	list_for_each_entry_safe(sb, n, &super_blocks, s_list) {
 		if (list_empty(&sb->s_instances))
 			continue;
 		if (sb->s_nr_dentry_unused == 0)
@@ -592,14 +591,10 @@ static void prune_dcache(int count)
 		}
 		spin_lock(&sb_lock);
 		count -= pruned;
-		/*
-		 * restart only when sb is no longer on the list and
-		 * we have more work to do.
-		 */
-		if (__put_super_and_need_restart(sb) && count > 0) {
-			spin_unlock(&sb_lock);
-			goto restart;
-		}
+		__put_super(sb);
+		/* more work left to do? */
+		if (count <= 0)
+			break;
 	}
 	spin_unlock(&sb_lock);
 	spin_unlock(&dcache_lock);