Multiple APIs not thread-safe

[Tim Burke]

According to the docstring [0], liberasurecode_backend_instance_get_by_desc() expects callers to already hold active_instances_rwlock, but only liberasurecode_backend_instance_register() and liberasurecode_backend_instance_unregister() do. As a result, the following callers may try to read the data structure while it's still being updated in another thread:

- liberasurecode_instance_destroy(), before it calls liberasurecode_backend_instance_unregister()
- liberasurecode_encode_cleanup()
- liberasurecode_encode()
- liberasurecode_decode_cleanup()
- liberasurecode_decode()
- liberasurecode_reconstruct_fragment()
- liberasurecode_fragments_needed()
- is_invalid_fragment_metadata()
- is_invalid_fragment()
- liberasurecode_get_aligned_data_size()
- liberasurecode_get_fragment_size()
- liberasurecode_verify_stripe_metadata() (indirectly, via is_invalid_fragment_metadata() and is_invalid_fragment())

[0] https://opendev.org/openstack/liberasurecode/src/tag/1.6.2/src/erasurecode.c#L83-L97