MAINTAINERS: update idmapping tree

Since Seth joined as a maintainer in ba40a57f ("Add Seth Forshee as
co-maintainer for idmapped mounts") it was best to get a shared git tree
instead of using our personal repositories. So we requested and
Konstantin suggested and gave us a new "idmapping" repository under the
pre-existing but mainly unused vfs namespace. Just makes it easier for
Seth to send fixes in case I'm out or someone else ever takes over.

Cc: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
Link: https://lore.kernel.org/r/20220816113514.43304-2-brauner@kernel.org