block: Set BIO_TRACE_COMPLETION on new bio during split

We inadvertently set it again on the source bio, but we need
to set it on the new split bio instead.

Fixes: fbbaf700 ("block: trace completion of all bios.")
Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>