Fix data race setting `default_DSO_meth`

Fix data race setting `default_DSO_meth`

The global variable `default_DSO_meth` was potentially set multiple times by
different threads.  It turns out that it could only be set to a single value
so the race is harmless but still better avoided.  The fix here simply removes
the global and accesses the value it was set to via the `DSO_METHOD_openssl()`
call.

Problem discovered via #16970, but this does not resolve that issue because
there are other concerns.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16972)