DERBY-6503: Ignore LinkageErrors when loading FilePermissionServiceImpl

When attempting to load the class on Java 6, an
UnsupportedClassVersionError is (correctly) raised, but it is not
among the exceptions that are caught and ignored. Add a catch clause
for its superclass LinkageError to handle it.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1580845 13f79535-47bb-0310-9956-ffa450edef68