kernfs: Refactor kernfs_get_open_node()

Factor out commont part. This is cleaner and should help with future
changes. No functional changes.

Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Reviewed-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20220828050440.734579-4-tj@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>