RFR: 8357959: (bf) ByteBuffer.allocateDirect initialization can result in large TTSP spikes

[Rohitash Kumar]

ByteBuffer.allocateDirect uses UNSAFE.setMemory, causing high time-to-safepoint (100+ ms) for large (100 MB+) allocations.

This PR applies a simple fix by chunking the zeroing operation within ByteBuffers. A more robust solution would be to add chunking inside UNSAFE.setMemory itself. However Its that straightforward as mentionedby Aleksey in [JDK-8357959](https://bugs.openjdk.org/browse/JDK-8357959)
>Looks like all current uses we care about are in Buffers. Taking a safepoint within cleaning would open some questions whether any VM code expect to see semi-initialized area we are busy cleaning up. For Buffers, this question does not arise. Therefore, we can do the fix in Buffers first, without changing the Unsafe itself.
 
I can pursue that if that is preferred. I chose 1 MB as a chunk size some what arbitrarily I am open to suggestion, if there are better options.

I added a benchmark for potential perf regression, following are the results.

**Before**

Benchmark                                              (bytes)  Mode  Cnt          Score         Error  Units
ByteBufferAllocationBenchmark.allocateDirectBuffer         128  avgt   25        860.869 ±     170.921  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer        1024  avgt   25       1368.599 ±     320.159  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer     1048576  avgt   25      80169.230 ±     448.075  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer  2147483647  avgt   25  635047372.458 ± 5461328.161  ns/op

**After**

Benchmark                                              (bytes)  Mode  Cnt          Score         Error  Units
ByteBufferAllocationBenchmark.allocateDirectBuffer         128  avgt   25        869.676 ±     160.633  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer        1024  avgt   25       1328.623 ±     282.198  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer     1048576  avgt   25      80224.157 ±     454.906  ns/op
ByteBufferAllocationBenchmark.allocateDirectBuffer  2147483647  avgt   25  667037308.198 ± 4818648.535  ns/op

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

Commit messages:
 - change output timeunit to nanoseconds
 - add benchmark for allocation
 - zero out memory in chunk size of 1MB

Changes: https://git.openjdk.org/jdk/pull/25487/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25487&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357959
  Stats: 69 lines in 2 files changed: 67 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25487.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25487/head:pull/25487

PR: https://git.openjdk.org/jdk/pull/25487