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/nfs4xdr.c b/fs/nfsd/nfs4xdr.c
index cab9780..a8587e9 100644
--- a/fs/nfsd/nfs4xdr.c
+++ b/fs/nfsd/nfs4xdr.c
@@ -2196,11 +2196,14 @@
 	 * we will not follow the cross mount and will fill the attribtutes
 	 * directly from the mountpoint dentry.
 	 */
-	if (d_mountpoint(dentry) && !attributes_need_mount(cd->rd_bmval))
-		ignore_crossmnt = 1;
-	else if (d_mountpoint(dentry)) {
+	if (nfsd_mountpoint(dentry, exp)) {
 		int err;
 
+		if (!(exp->ex_flags & NFSEXP_V4ROOT)
+				&& !attributes_need_mount(cd->rd_bmval)) {
+			ignore_crossmnt = 1;
+			goto out_encode;
+		}
 		/*
 		 * Why the heck aren't we just using nfsd_lookup??
 		 * Different "."/".." handling?  Something else?
@@ -2216,6 +2219,7 @@
 			goto out_put;
 
 	}
+out_encode:
 	nfserr = nfsd4_encode_fattr(NULL, exp, dentry, p, buflen, cd->rd_bmval,
 					cd->rd_rqstp, ignore_crossmnt);
 out_put: