nfsd: filter readdir results in V4ROOT case

As with lookup, we treat every boject as a mountpoint and pretend it
doesn't exist if it isn't exported.

The preexisting code here is confusing, but I haven't yet figured out
how to make it clearer.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/vfs.h b/fs/nfsd/vfs.h
index b8011fd..f4fa6d3 100644
--- a/fs/nfsd/vfs.h
+++ b/fs/nfsd/vfs.h
@@ -40,6 +40,7 @@
 				struct svc_export **, struct dentry **);
 __be32		nfsd_setattr(struct svc_rqst *, struct svc_fh *,
 				struct iattr *, int, time_t);
+int nfsd_mountpoint(struct dentry *, struct svc_export *);
 #ifdef CONFIG_NFSD_V4
 __be32          nfsd4_set_nfs4_acl(struct svc_rqst *, struct svc_fh *,
                     struct nfs4_acl *);