Add Seth Forshee as co-maintainer for idmapped mounts

Seth has been integral in the design and implementation of idmapped
mounts and was the main architect behind the s_user_ns work which
ultimately made filesystems such as FUSE and overlayfs available in
containers. He continues to be active in both development and review.
I'm very happy he decided to maintain this feature. He has my full
trust.

Link: https://lore.kernel.org/r/20220726141615.1046027-1-brauner@kernel.org
Cc: Seth Forshee <sforshee@digitalocean.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>