MAINTAINERS: NFSD should be responsible for fs/exportfs

We recently received a patch for fs/exportfs/expfs.c, but there
isn't a subsystem maintainer listed for fs/exportfs:

Christian Brauner <brauner@kernel.org> (commit_signer:2/2=100%,authored:1/2=50%,added_lines:3/6=50%,removed_lines:2/6=33%)
Al Viro <viro@zeniv.linux.org.uk> (commit_signer:1/2=50%,authored:1/2=50%,added_lines:3/6=50%,removed_lines:4/6=67%)
Miklos Szeredi <mszeredi@redhat.com> (commit_signer:1/2=50%)
Amir Goldstein <amir73il@gmail.com> (commit_signer:1/2=50%)
linux-kernel@vger.kernel.org (open list)

Neil says:
> Looking at recent commits, patches come in through multiple
> different trees.
> nfsd certainly has an interest in expfs.c.  The only other user is
> name_to_handle/open_by_handle API.
> I see it as primarily nfsd functionality which is useful enough to
> be exported directly to user-space.
> (It was created by me when I was nfsd maintainer - does that
> count?)

Suggested-by: Neil Brown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Acked-by: Jeff Layton <jlayton@kernel.org>