RFR: 8274632: Possible pointer overflow in PretouchTask chunk claiming

[Kim Barrett]

Please review this change to PretouchTask, fixing the mechanism used to
claim chunks for parallel touching.  The old mechanism unconditionally added
a potentially large value to the current pointer and then checked the result
for overflow.  But since pointer arithmetic overflow is UB, that's not a
reliable approach.

Testing:
mach5 tier1

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Commit messages:
 - use cmpxchg to claim chunks

Changes: https://git.openjdk.java.net/jdk/pull/5791/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5791&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274632
  Stats: 8 lines in 1 file changed: 1 ins; 2 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5791.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5791/head:pull/5791

PR: https://git.openjdk.java.net/jdk/pull/5791