MAINTAINERS: Make Yu Kuai co-maintainer of md/raid subsystem

In the past couple years, Yu Kuai has been making solid contributions
to md/raid subsystem. Make Yu Kuai a co-maintainer.

Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20241108014112.2098079-1-song@kernel.org


Signed-off-by: Song Liu <song@kernel.org>