RFR: 8266055: ZGC: ZHeap::print_extended_on() doesn't disable deferred delete

[Per Liden]

In `ZHeap::print_extended_on()`, the last call to `_page_allocator.enable_deferred_delete()` should be `_page_allocator.disable_deferred_delete()`.

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Commit messages:
 - 8266055: ZGC: ZHeap::print_extended_on() doesn't disable deferred delete

Changes: https://git.openjdk.java.net/jdk/pull/3717/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3717&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266055
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3717.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3717/head:pull/3717

PR: https://git.openjdk.java.net/jdk/pull/3717