MAINTAINERS: create an entry for exportfs

Split the exportfs entry from the nfsd entry and add myself as reviewer.

Suggested-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231026205553.143556-1-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>