Fix assertion in test-mono-linked-list-set

When running 'make check', test-mono-linked-list-set fails with:

* Assertion at hazard-pointer.c:353, condition `small_id == i' not met

This happens since 71ad74dc11c5fa4bc8c178a5457d2cab732fdb01, because
that commit changes the behaviour of mono_thread_smr_init() so that
invoking it should not be invoked more than once.
Merge pull request #988 from ranma42/fix-lls-test

Fix assertion in test-mono-linked-list-set