rbd: update MAINTAINERS info

Alex has got plenty on his plate aside from rbd and hasn't really been
active in recent years.  Remove his maintainership entry.

Dongsheng is very familiar with the code base and has been reviewing rbd
patches for a while now.  Add him as a reviewer.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Acked-by: Alex Elder <elder@kernel.org>
Acked-by: Dongsheng Yang <dongsheng.yang@easystack.cn>