This fix updates swarmkit to 5a6df4b07d83e6dbd72e39e354c325dc9b91850f.
This fix is needed by #29074 (docker PR) and is related to
docker/swarmkit#1789 (swarmkit PR) and #29044
This fix may be needed for 1.13.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>