Fix loopargs_t object duplication into ASYNC context

Fix loopargs_t object duplication into ASYNC context

Code was relying on an implicit data-sharing through duplication of
loopargs_t pointer-members made by ASYNC_start_job().

Now share structure address instead of structure content.

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>