Remove the isinited variable from child_prov_globals

Remove the isinited variable from child_prov_globals

This variable might have made sense at some point but it not longer does
so. It was being used to check whether we are still initing or not. If we
are still initing then the assumption was that we already hold the lock.
That assumption was untrue. We need to always take the lock.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16980)