fuse: try to fix mount errors on OSX

Some of the error codes due to the poll hack get returned back to the
OSX mount process, which leads to failing mounts.

Change-Id: I5b10509c3b8173343caf3f67c0e12a25ddfae32b

fuse/poll.go

@@ -34,9 +34,16 @@ func doPollHackLookup(ms *Server, req *request) {
 		req.status = OK
 	case _OP_POLL:
 		req.status = ENOSYS
+
+	case _OP_ACCESS, _OP_FLUSH, _OP_RELEASE:
+		// Avoid upsetting the OSX mount process.
+		req.status = OK
 	default:
 		// We want to avoid switching off features through our
-		// poll hack, so don't use ENOSYS
+		// poll hack, so don't use ENOSYS. It would be nice if
+		// we could transmit no error code at all, but for
+		// some opcodes, we'd have to invent credible data to
+		// return as well.
 		req.status = ERANGE
 	}
 }