ksmbd: Replace one-element arrays with flexible-array members

Replace the deprecated one-element arrays with flexible-array members
in the structs filesystem_attribute_info and filesystem_device_info.

There are no binary differences after this conversion.

Link: https://github.com/KSPP/linux/issues/79


Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>