git: openjdk/loom: fibers: Fix keepalive for preempt

[duke]

Changeset: f667afe3
Author:    Rickard Bäckman <rickard.backman at oracle.com>
Date:      2019-12-03 10:55:47 +0000
URL:       https://git.openjdk.java.net/loom/commit/f667afe3

Fix keepalive for preempt

! src/hotspot/share/runtime/continuation.cpp