fusefs: fix the last_local_modify LLM/LastLocalModify.lookup/3 test

The LastLocalModify tests were originally written to simulate a race
condition between VOP_SETATTR and VOP_LOOKUP. They were later extended
to cover some other VOPs that can affect file size, including VOP_WRITE.
However, the test never correctly simulated the race with VOP_WRITE. So
that test only ever passed by accident. Fix it by always opening the
file with O_DIRECT.

PR: 289237
Reported by: Siva Mahadevan <me@svmhdvn.name>
MFC after: 1 week