RFR: Rename nullfree to null_free

[David Simms]

Renamed nullfree to null_free

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Commit messages:
 - Rename nullfree to null_free

Changes: https://git.openjdk.java.net/valhalla/pull/416/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=416&range=00
  Stats: 46 lines in 12 files changed: 0 ins; 2 del; 44 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/416.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/416/head:pull/416

PR: https://git.openjdk.java.net/valhalla/pull/416