MAINTAINERS: exclude 9p from networking

We don't have much to say about 9p, even tho it lives under net/.
Avoid CCing netdev.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>