cgroup: export fhandle info for a cgroup

Add an API to export cgroup fhandle info. We don't export a full 'struct
file_handle', there are unrequired info. Sepcifically, cgroup is always
a directory, so we don't need a 'FILEID_INO32_GEN_PARENT' type fhandle,
we only need export the inode number and generation number just like
what generic_fh_to_dentry does. And we can avoid the overhead of getting
an inode too, since kernfs_node_id (ino and generation) has all the info
required.

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/cgroup.h

@@ -609,6 +609,10 @@ static inline void cgroup_kthread_ready(void)
 	current->no_cgroup_migration = 0;
 }
 
+static inline union kernfs_node_id *cgroup_get_kernfs_id(struct cgroup *cgrp)
+{
+	return &cgrp->kn->id;
+}
 #else /* !CONFIG_CGROUPS */
 
 struct cgroup_subsys_state;
@@ -631,6 +635,10 @@ static inline int cgroup_init_early(void) { return 0; }
 static inline int cgroup_init(void) { return 0; }
 static inline void cgroup_init_kthreadd(void) {}
 static inline void cgroup_kthread_ready(void) {}
+static inline union kernfs_node_id *cgroup_get_kernfs_id(struct cgroup *cgrp)
+{
+	return NULL;
+}
 
 static inline bool task_under_cgroup_hierarchy(struct task_struct *task,
 					       struct cgroup *ancestor)