mm: dirty balancing for tasks

Based on ideas of Andrew:
  http://marc.info/?l=linux-kernel&m=102912915020543&w=2

Scale the bdi dirty limit inversly with the tasks dirty rate.
This makes heavy writers have a lower dirty limit than the occasional writer.

Andrea proposed something similar:
  http://lwn.net/Articles/152277/

The main disadvantage to his patch is that he uses an unrelated quantity to
measure time, which leaves him with a workload dependant tunable. Other than
that the two approaches appear quite similar.

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/sched.h

@@ -74,6 +74,7 @@ struct sched_param {
 #include <linux/pid.h>
 #include <linux/percpu.h>
 #include <linux/topology.h>
+#include <linux/proportions.h>
 #include <linux/seccomp.h>
 #include <linux/rcupdate.h>
 #include <linux/futex.h>
@@ -1149,6 +1150,7 @@ struct task_struct {
 #ifdef CONFIG_FAULT_INJECTION
 	int make_it_fail;
 #endif
+	struct prop_local_single dirties;
 };
 
 /*