MAINTAINERS: update nilfs2 entry

MAINTAINERS: update nilfs2 entry

Viacheslav has kindly offered to help with the maintenance of nilfs2 by
upstreaming patches, similar to the HFS/HFS+ tree.  I've accepted his
offer, and will therefore add him as a co-maintainer and switch the
project's git tree for that role.

At the same time, change the outdated status field to Maintained to
reflect the current state.

Link: https://lkml.kernel.org/r/20251107153530.9023-1-konishi.ryusuke@gmail.com
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Acked-by: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>