sched/debug: Dump domains' sched group flags

There have been a case where the SD_SHARE_CPUCAPACITY sched group flag
in a parent domain were not set and propagated properly when a degenerate
domain is removed.

Add dump of domain sched group flags of a CPU to make debug easier
in the future.

Usage:
cat /debug/sched/domains/cpu0/domain1/groups_flags
to dump cpu0 domain1's sched group flags.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/ed1749262d94d95a8296c86a415999eda90bcfe3.1688770494.git.tim.c.chen@linux.intel.com

kernel/sched/debug.c

@@ -427,6 +427,7 @@ static void register_sd(struct sched_domain *sd, struct dentry *parent)
 #undef SDM
 
 	debugfs_create_file("flags", 0444, parent, &sd->flags, &sd_flags_fops);
+	debugfs_create_file("groups_flags", 0444, parent, &sd->groups->flags, &sd_flags_fops);
 }
 
 void update_sched_domain_debugfs(void)