[sh/11] 8239926: Shenandoah: Shenandoah needs to mark nmethod's metadata

[Roman Kennke]

>> Original Bug: https://bugs.openjdk.java.net/browse/JDK-8239926
>> sh/11u Webrev: 
>> http://cr.openjdk.java.net/~zgu/JDK-8239926/sh-jdk11/webrev.00/
> 
> Backport looks fine. Is your intent to push this for April CPU? If so, it needs to go in as soon as
> possible. Preferably before I pull 11.0.7+9 tomorrow. Roman needs to make that call.

Hmm yeah, it seems to fix a rare bug that can be disastrous (heap
corruption), so I would include it. The change looks good to me.

Thank you,
Roman