sunrpc: use kmemdup_nul() in gssp_stringify()

It is more efficient to use kmemdup_nul() if the size is known exactly
.

According to doc:
"Note: Use kmemdup_nul() instead if the size is known exactly."

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>