MAINTAINERS: Update RDMA migration entry with M:

We wanted to remove RDMA migration once but that didn't go further.  In
reality, with the help of Zhijian it's in Odd Fixes stage, even if we just
merged one new parameter for it, for performance improvements.

Markus pointed out we'd better have at least one M: for it to match
anything that is not orphaned.

Remove the X: for Migration entry for RDMA files, then it'll start to cover
RDMA migration again. Keep the separate entry so Zhijian can keep getting
copied, and copy the M:s over to say someone is collecting patches.
Logically these M:s aren't needed after removing X:, but make it clearer.

Link: https://lore.kernel.org/r/5326b854-fcea-4af6-a479-792888a94a4d@fujitsu.com
Cc: Zhijian Li (Fujitsu) <lizhijian@fujitsu.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-ID: <20260511143026.1296485-1-peterx@redhat.com>
Acked-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>