[foreign-jextract] RFR: Minor change for possible race in shared resource list

[Radoslaw Smogura]

(1) One or more threads can see "prev" as CLOSED_LIST. Even those CAS will be performed
and list head will be set to new cleanup. Than exception is going to be thrown.

(2) Than cleanup will continue (maybe skipping just added cleanup).

If in addition, other thread will join in parallel to (1) with cleanup2, than
queue could look like this HEAD = cleanup2 -> list end marker.

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Commit messages:
 - This is minor change for possible race.

Changes: https://git.openjdk.java.net/panama-foreign/pull/519/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=519&range=00
  Stats: 9 lines in 1 file changed: 4 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/519.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/519/head:pull/519

PR: https://git.openjdk.java.net/panama-foreign/pull/519