alru: add parameter `max_dirty_ratio'

With a high dirty ratio and occupancy, OCF might unable to map cache lines
for new requests, thus pass-through the I/O to core devices.  IOPS will
drop afterwards.  We need to control the dirty ratio.

Existing `alru' policy gives user the chance to control the stale buffer
time, activity threshold etc.  They can affect the dirty ratio of the cache
device, but in an empirical manner, more or less.  Introducing
`max_dirty_ratio' can make it explicit.

At first glance, it might be better to implement a dedicated cleaner policy
directly targeting dirty ratio goal, so that the `alru' parameters remains
orthogonal.  But one the other hand, we still need to flush dirty cache
lines periodically, instead of just keeping a watermark of dirty ratio.
It indicates that existing `alru' parameters are still required if we
develop a new policy, and it seems reasonable to make it a parameter.

To sum up, this patch does the following:
- added a 'max_dirty_ratio' parameter with default value 100;
- with default value 100, `alru' cleaner is identical to what is was;
- with value N less than 100, the cleaner (when waken up) will active
  brought dirty ratio to N, regardless of staleness time.

Signed-off-by: David Lee <live4thee@gmail.com>

inc/cleaning/alru.h

@@ -15,6 +15,7 @@ enum ocf_cleaning_alru_parameters {
 	ocf_alru_stale_buffer_time,
 	ocf_alru_flush_max_buffers,
 	ocf_alru_activity_threshold,
+	ocf_alru_max_dirty_ratio,
 };
 
 /**
@@ -66,6 +67,16 @@ enum ocf_cleaning_alru_parameters {
 /** Idle time before flushing thread can start default value */
 #define OCF_ALRU_DEFAULT_ACTIVITY_THRESHOLD	10000
 
+/**
+ * ALRU max dirty ratio for a cache device
+ */
+
+/** Minimum dirty ratio value */
+#define OCF_ALRU_MIN_MAX_DIRTY_RATIO		0
+/** Maximum dirty ratio value */
+#define OCF_ALRU_MAX_MAX_DIRTY_RATIO		100
+/** Default dirty ratio value */
+#define OCF_ALRU_DEFAULT_MAX_DIRTY_RATIO	OCF_ALRU_MAX_MAX_DIRTY_RATIO
 /**
  * @}
  */