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

MAINTAINERS

@@ -9780,7 +9780,7 @@ M:	Christian Brauner <brauner@kernel.org>
 M:	Seth Forshee <sforshee@kernel.org>
 L:	linux-fsdevel@vger.kernel.org
 S:	Maintained
-T:	git git://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git
+T:	git://git.kernel.org/pub/scm/linux/kernel/git/vfs/idmapping.git
 F:	Documentation/filesystems/idmappings.rst
 F:	tools/testing/selftests/mount_setattr/
 F:	include/linux/mnt_idmapping.h