Ensure |rwstate| is set correctly on BIO_flush

A BIO_flush call in the DTLS code was not correctly setting the |rwstate|
variable to SSL_WRITING. This means that SSL_get_error() will not return
SSL_ERROR_WANT_WRITE in the event of an IO retry.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(cherry picked from commit 67f60be8)