NFSD: Teach nfsd_mountpoint() auto mounts

Currently nfsd_mountpoint() tests for mount points using d_mountpoint(),
this works only when a mount point is already uncovered.
In our case the mount point is of type auto mount and can be coverted.
i.e. ->d_automount() was not called.

Using d_managed() nfsd_mountpoint() can test whether a mount point is
either already uncovered or can be uncovered later.
Signed-off-by: Richard Weinberger <richard@nod.at>
Reviewed-by: Ian Kent <raven@themaw.net>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/vfs.c

@@ -223,7 +223,7 @@ int nfsd_mountpoint(struct dentry *dentry, struct svc_export *exp)
 		return 1;
 	if (nfsd4_is_junction(dentry))
 		return 1;
-	if (d_mountpoint(dentry))
+	if (d_managed(dentry))
 		/*
 		 * Might only be a mountpoint in a different namespace,
 		 * but we need to check.