ZEN: Restore dirty (background) ratios to mainline default

As recently discovered in an issue with Zen Kernel swapping excessively
with mgLRU on [1], the high dirty ratio values configured with
ZEN_INTERACTIVE cause this new LRU feature to misbehave.

Even if there's an issue with the implementation of Multigenerational
LRU itself, using a high dirty ratio does not make sense anymore in
2021.  When the high ratios were initially selected, most systems core
drive was spinning rust.  Now-a-days, nearly all systems are sold with a
solid state drive with some systems having hard drives for non essential
storage or backups.

Not to mention, the storage subsystem completely changed with the
introduction of blkmq [2], and interactivity optimized IO schedulers
like BFQ matured and went mainline.  With all of the above, there's no
reason to keep the our custom ratios any longer.

[1] #216
[2] https://www.kernel.org/doc/html/latest/block/blk-mq.html

init/Kconfig

@@ -116,8 +116,6 @@ config ZEN_INTERACTIVE
 
 	  --- Virtual Memory Subsystem ---------------------------
 
-	    Mem dirty before bg writeback..:  10 %  ->  20 %
-	    Mem dirty before sync writeback:  20 %  ->  50 %
 	    Background-reclaim hugepages...:    no  -> yes
 
 	  --- CFS CPU Scheduler ----------------------------------

mm/page-writeback.c

@@ -71,11 +71,7 @@ static long ratelimit_pages = 32;
 /*
  * Start background writeback (via writeback threads) at this percentage
  */
-#ifdef CONFIG_ZEN_INTERACTIVE
-int dirty_background_ratio = 20;
-#else
 int dirty_background_ratio = 10;
-#endif
 
 /*
  * dirty_background_bytes starts at 0 (disabled) so that it is a function of
@@ -92,11 +88,7 @@ int vm_highmem_is_dirtyable;
 /*
  * The generator of dirty data starts writeback at this percentage
  */
-#ifdef CONFIG_ZEN_INTERACTIVE
-int vm_dirty_ratio = 50;
-#else
 int vm_dirty_ratio = 20;
-#endif
 
 /*
  * vm_dirty_bytes starts at 0 (disabled) so that it is a function of