test_librbd_fsx: use private RNG context

It is at the core of fsx to be able to reproduce the exact op sequence
that lead to a failure.  Use reentrant glibc RNG functions to make sure
that nothing else has to chance to mess up the sequence.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #2038 from ceph/wip-fsx-random-ctx

test_librbd_fsx: use private RNG context

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>