RFR: 8270100: Fix some inaccurate GC logging

[Volker Simonis]

If running with `-Xlog:gc+heap*=trace` the JVM will print the extra per thread amount which is added to the new generation on resize:

[0,105s][debug][gc,ergo,heap ] GC(0) New generation size 34112K->34176K [eden=27392K,survivor=3392K]
[0,105s][trace][gc,ergo,heap ] GC(0) [allowed 0K extra for 0 threads]

Currently this will always print "0K extra for 0 threads" no matter how much extra space was added.

Also, the shrink factor will always be printed to be 0%, even if we run with `-XX:-ShrinkHeapInSteps` which pins the shrink factor at 100%:

[13,213s][trace][gc,heap ] GC(34) shrink_bytes: 463564,0K current_shrink_factor: 0 new shrink factor: 0 _min_heap_delta_bytes: 192,0K
[13,239s][trace][gc,heap ] GC(34) Shrinking tenured generation from 531852K to 68288K


The fix is trivial.

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

Commit messages:
 - 8270100: Fix some inaccurate GC logging

Changes: https://git.openjdk.java.net/jdk/pull/4727/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4727&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270100
  Stats: 16 lines in 3 files changed: 5 ins; 2 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4727.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4727/head:pull/4727

PR: https://git.openjdk.java.net/jdk/pull/4727