nfsd: ignore requests to disable unsupported versions

The kernel currently errors out if you attempt to enable or disable a
version that it doesn't recognize. Change it to ignore attempts to
disable an unrecognized version. If we don't support it, then there is
no harm in doing so.
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfsctl.c

@@ -601,7 +601,9 @@ static ssize_t __write_versions(struct file *file, char *buf, size_t size)
 				}
 				break;
 			default:
-				return -EINVAL;
+				/* Ignore requests to disable non-existent versions */
+				if (cmd == NFSD_SET)
+					return -EINVAL;
 			}
 			vers += len + 1;
 		} while ((len = qword_get(&mesg, vers, size)) > 0);