Pass fragment to dtls1_retransmit_message.

We can pass the fragment pointer to dtls1_retransmit_message rather than
having it look it up again.

Change-Id: If6957428418a44e7ceac91a93f7c6032d331d9d8
Reviewed-on: https://boringssl-review.googlesource.com/3510
Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>