MAINTAINERS: configfs: split configfs entry in C and Rust parts

Split configfs MAINTAINERS entry in C and Rust parts. Mark Breno Leitao as
maintainer of configfs C parts. Mark Andreas Hindborg as maintainer of
configfs Rust parts.

Acked-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>