Make allocation of anon devices cheaper

Standard trick - add a new variable (start) such that
for each n < start n is known to be busy.  Allocation can
skip checking everything in [0..start) and if it returns
n, we can set start to n + 1.  Freeing below start sets
start to what we'd just freed.

Of course, it still sucks if we do something like
	free 0
	allocate
	allocate
in a loop - still O(n^2) time.  However, on saner loads it
improves the things a lot and the entire thing is not worth
the trouble of switching to something with better worst-case
behaviour.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/super.c

@@ -608,6 +608,7 @@ void emergency_remount(void)
 
 static DEFINE_IDA(unnamed_dev_ida);
 static DEFINE_SPINLOCK(unnamed_dev_lock);/* protects the above */
+static int unnamed_dev_start = 0; /* don't bother trying below it */
 
 int set_anon_super(struct super_block *s, void *data)
 {
@@ -618,7 +619,8 @@ int set_anon_super(struct super_block *s, void *data)
 	if (ida_pre_get(&unnamed_dev_ida, GFP_ATOMIC) == 0)
 		return -ENOMEM;
 	spin_lock(&unnamed_dev_lock);
-	error = ida_get_new(&unnamed_dev_ida, &dev);
+	error = ida_get_new_above(&unnamed_dev_ida, unnamed_dev_start, &dev);
+	unnamed_dev_start = dev + 1;
 	spin_unlock(&unnamed_dev_lock);
 	if (error == -EAGAIN)
 		/* We raced and lost with another CPU. */
@@ -629,6 +631,7 @@ int set_anon_super(struct super_block *s, void *data)
 	if ((dev & MAX_ID_MASK) == (1 << MINORBITS)) {
 		spin_lock(&unnamed_dev_lock);
 		ida_remove(&unnamed_dev_ida, dev);
+		unnamed_dev_start = dev;
 		spin_unlock(&unnamed_dev_lock);
 		return -EMFILE;
 	}
@@ -645,6 +648,8 @@ void kill_anon_super(struct super_block *sb)
 	generic_shutdown_super(sb);
 	spin_lock(&unnamed_dev_lock);
 	ida_remove(&unnamed_dev_ida, slot);
+	if (slot < unnamed_dev_start)
+		unnamed_dev_start = slot;
 	spin_unlock(&unnamed_dev_lock);
 }