kernfs_root_from_sb
root = kernfs_root_from_sb(dentry->d_sb);
struct kernfs_root *kernfs_root_from_sb(struct super_block *sb);
struct kernfs_root *kf_root = kernfs_root_from_sb(fc->root->d_sb);
struct kernfs_root *kf_root = kernfs_root_from_sb(sb);