kernfs: remove an unused if statement in kernfs_path_from_node_locked()

It makes no sense to call kernfs_path_from_node_locked() with NULL buf,
and no one is doing that right now.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20221126111634.1994-1-thunder.leizhen@huawei.com


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