RFR: 8254871: Remove unnecessary string copy in NetworkInterface.c

[Eric Liu]

A small improvement to avoid extra string copy.

[Tests]
Jtreg hotspot::hotspot_all_no_apps, jdk::jdk_core and langtools::tier1.
No new failure found.

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Commit messages:
 - 8254871: Remove unnecessary string copy in NetworkInterface.c

Changes: https://git.openjdk.java.net/jdk/pull/821/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=821&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254871
  Stats: 8 lines in 1 file changed: 0 ins; 5 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/821.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/821/head:pull/821

PR: https://git.openjdk.java.net/jdk/pull/821