cgtop: use common function to query cgroup root

show_cgroup_get_root_and_warn is renamed to show_cgroup_get_path_and_warn
because it now optionally allows querying a non-root path.

This removes duplicated code and teaches cgtop to combine
-M with a root prefix:

$ systemd-cgtop -M myprecious /system.slice
...

1 files changed, 2 insertions, 1 deletions

diff --git a/src/shared/cgroup-show.h b/src/shared/cgroup-show.h
index 0574f4055..1445c05f1 100644
--- a/src/shared/cgroup-show.h
+++ b/src/shared/cgroup-show.h
@@ -31,6 +31,7 @@ int show_cgroup(const char *controller, const char *path, const char *prefix, un
 int show_cgroup_and_extra_by_spec(const char *spec, const char *prefix, unsigned n_columns, const pid_t extra_pids[], unsigned n_extra_pids, OutputFlags flags);
 int show_cgroup_and_extra(const char *controller, const char *path, const char *prefix, unsigned n_columns, const pid_t extra_pids[], unsigned n_extra_pids, OutputFlags flags);
 
-int show_cgroup_get_root_and_warn(
+int show_cgroup_get_path_and_warn(
                 const char *machine,
+                const char *prefix,
                 char **ret);