Restore sensible "sess_accept" counter tracking
Commit 9ef9088c switched the SSL/SSL_CTX statistics counters to using Thread-Sanitizer-friendly primitives. However, it erroneously converted an addition of -1 (for s->session_ctx->stats.sess_accept) to an addition of +1, since that is the only counter API provided by the internal tsan_assist.h header until the previous commit. This means that for each accepted (initial) connection, the session_ctx's counter would get doubly incremented, and the (switched) ctx's counter would also get incremented. Restore the counter decrement so that each accepted connection increments exactly one counter exactly once (in net effect). Reviewed-by:Paul Dale <paul.dale@oracle.com> (Merged from https://github.com/openssl/openssl/pull/7464)