RFR: 8285710: miscalculation of G1CardSetAllocator wasted memory size

[tqxia]

when calculating the wasted memory size of G1CardSetAllocator, the code erroneously substracted both _segmented_array.num_allocated_slots() and _free_slots_list.pending_count() from _segmented_array.num_available_slots().

The correct formula should be: num_wasted_slots = _segmented_array.num_available_slots() - (_segmented_array.num_allocated_slots() - (uint)_free_slots_list.pending_count()).

This can potentially leads to an arithmetic overflow and misleading information will be displayed when G1SummarizeRSetStatsPeriod is set.

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Commit messages:
 - 8285710: miscalculation of G1CardSetAllocator wasted memory size

Changes: https://git.openjdk.java.net/jdk/pull/8424/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8424&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285710
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8424.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8424/head:pull/8424

PR: https://git.openjdk.java.net/jdk/pull/8424