RFR: 8278115: gc/stress/gclocker/TestGCLockerWithSerial.java has duplicate -Xmx

[Aleksey Shipilev]

Spotted by Andrey here:
 https://github.com/openjdk/jdk/pull/6574#issuecomment-983959564

Seems to be that way all the way back to JDK-8177968. Should be -Xms, like other GCLocker tests. 

Additional testing:
 - [x] Affected test still passes on Linux x86_64 fastdebug

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Commit messages:
 - Fix

Changes: https://git.openjdk.java.net/jdk/pull/6659/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6659&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278115
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6659.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6659/head:pull/6659

PR: https://git.openjdk.java.net/jdk/pull/6659