nfsd4: cast readlink() bug argument

As we already do in readv, writev.
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/vfs.c

@@ -1581,7 +1581,7 @@ nfsd_readlink(struct svc_rqst *rqstp, struct svc_fh *fhp, char *buf, int *lenp)
 	 */
 
 	oldfs = get_fs(); set_fs(KERNEL_DS);
-	host_err = inode->i_op->readlink(path.dentry, buf, *lenp);
+	host_err = inode->i_op->readlink(path.dentry, (char __user *)buf, *lenp);
 	set_fs(oldfs);
 
 	if (host_err < 0)