RFR: 8314071: Test java/foreign/TestByteBuffer.java timed out

[Per Minborg]

This PR proposes improving performance for two `VarHandle` objects and increasing the timeout for the tests. The size of a mapped file is also reduced from 3 GiB to 2.5 GiB. This still crosses the 2<sup> 31</sup> boundary.

-------------

Commit messages:
 - Improve performance and increase timeout

Changes: https://git.openjdk.org/jdk/pull/15326/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15326&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314071
  Stats: 12 lines in 1 file changed: 0 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/15326.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15326/head:pull/15326

PR: https://git.openjdk.org/jdk/pull/15326