relay: fix bogus cast in subbuf_splice_actor()

The current code that sets the read position in subbuf_splice_actor may
give erroneous results if the buffer size isn't a power of 2.  This
patch fixes the problem.

Signed-off-by: Tom Zanussi <zanussi@us.ibm.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>