kernfs: implement kernfs_node_from_dentry(), kernfs_root_from_sb() and kernfs_rename()

kernfs: implement kernfs_node_from_dentry(), kernfs_root_from_sb() and kernfs_rename()

Implement helpers to determine node from dentry and root from
super_block.  Also add a kernfs_rename_ns() wrapper which assumes NULL
namespace.  These generally make sense and will be used by cgroup.

v2: Some dummy implementations for !CONFIG_SYSFS was missing.  Fixed.
    Reported by kbuild test robot.

Signed-off-by: Tejun Heo <[email protected]>
Cc: kbuild test robot <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>