[-next v2] nvmet-auth: use kmemdup instead of kmalloc + memcpy

[Christoph Hellwig]

On Tue, Jul 26, 2022 at 10:56:32AM +0800, Zhang Xiaoxu wrote:
> For code neat purpose, we can use kmemdup to replace
> kmalloc + memcpy.

Thanks, applied to nvme-5.20 after fixing the coding style.