MAINTAINERS: ksmbd: switch Sergey to reviewer

Sergey don't have the time to work ksmbd. He will continue to review
ksmbd works at free time. This patch switches him from maintainer
to reviewer.

Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>