mm/damon/sysfs-schemes: skip stats update if the scheme directory is removed

A DAMON sysfs interface user can start DAMON with a scheme, remove the
sysfs directory for the scheme, and then ask update of the scheme's stats.
Because the schemes stats update logic isn't aware of the situation, it
results in an invalid memory access.  Fix the bug by checking if the
scheme sysfs directory exists.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0ac32b8 ("mm/damon/sysfs: support DAMOS stats")
Signed-off-by: SeongJae Park <[email protected]>
Cc: <[email protected]>	[v5.18]
Signed-off-by: Andrew Morton <[email protected]>

Author: sjp38

mm/damon/sysfs.c

@@ -2339,6 +2339,10 @@ static int damon_sysfs_upd_schemes_stats(struct damon_sysfs_kdamond *kdamond)
 	damon_for_each_scheme(scheme, ctx) {
 		struct damon_sysfs_stats *sysfs_stats;
 
+		/* user could have removed the scheme sysfs dir */
+		if (schemes_idx >= sysfs_schemes->nr)
+			break;
+
 		sysfs_stats = sysfs_schemes->schemes_arr[schemes_idx++]->stats;
 		sysfs_stats->nr_tried = scheme->stat.nr_tried;
 		sysfs_stats->sz_tried = scheme->stat.sz_tried;