mm/damon/core: add number of each enum type values

This commit declares the number of legal values for each DAMON enum types
to make traversals of such DAMON enum types easy and safe.

Link: https://lkml.kernel.org/r/20220228081314.5770-3-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Xin Hao <xhao@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: sjp38

include/linux/damon.h

@@ -87,6 +87,7 @@ struct damon_target {
  * @DAMOS_HUGEPAGE:	Call ``madvise()`` for the region with MADV_HUGEPAGE.
  * @DAMOS_NOHUGEPAGE:	Call ``madvise()`` for the region with MADV_NOHUGEPAGE.
  * @DAMOS_STAT:		Do nothing but count the stat.
+ * @NR_DAMOS_ACTIONS:	Total number of DAMOS actions
  */
 enum damos_action {
 	DAMOS_WILLNEED,
@@ -95,6 +96,7 @@ enum damos_action {
 	DAMOS_HUGEPAGE,
 	DAMOS_NOHUGEPAGE,
 	DAMOS_STAT,		/* Do nothing but only record the stat */
+	NR_DAMOS_ACTIONS,
 };
 
 /**
@@ -157,10 +159,12 @@ struct damos_quota {
  *
  * @DAMOS_WMARK_NONE:		Ignore the watermarks of the given scheme.
  * @DAMOS_WMARK_FREE_MEM_RATE:	Free memory rate of the system in [0,1000].
+ * @NR_DAMOS_WMARK_METRICS:	Total number of DAMOS watermark metrics
  */
 enum damos_wmark_metric {
 	DAMOS_WMARK_NONE,
 	DAMOS_WMARK_FREE_MEM_RATE,
+	NR_DAMOS_WMARK_METRICS,
 };
 
 /**