MAINTAINERS: Add Li Nan as md/raid reviewer

I've long contributed to and reviewed the md/raid subsystem. I've fixed
many bugs and done code refactors, with dozens of patches merged.
I now volunteer to work as a reviewer for this subsystem.

Link: https://lore.kernel.org/linux-raid/20260202083203.3017096-1-linan666@huaweicloud.com
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Yu Kuai <yukuai@fnnas.com>

Author: Li Nan

MAINTAINERS

@@ -24239,6 +24239,7 @@ F:	include/linux/property.h
 SOFTWARE RAID (Multiple Disks) SUPPORT
 M:	Song Liu <song@kernel.org>
 M:	Yu Kuai <yukuai@fnnas.com>
+R:	Li Nan <linan122@huawei.com>
 L:	linux-raid@vger.kernel.org
 S:	Supported
 Q:	https://patchwork.kernel.org/project/linux-raid/list/