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

kernel/relay.c

@@ -1074,7 +1074,9 @@ static int subbuf_splice_actor(struct file *in,
 	unsigned int pidx, poff, total_len, subbuf_pages, ret;
 	struct rchan_buf *rbuf = in->private_data;
 	unsigned int subbuf_size = rbuf->chan->subbuf_size;
-	size_t read_start = ((size_t)*ppos) % rbuf->chan->alloc_size;
+	uint64_t pos = (uint64_t) *ppos;
+	uint32_t alloc_size = (uint32_t) rbuf->chan->alloc_size;
+	size_t read_start = (size_t) do_div(pos, alloc_size);
 	size_t read_subbuf = read_start / subbuf_size;
 	size_t padding = rbuf->padding[read_subbuf];
 	size_t nonpad_end = read_subbuf * subbuf_size + subbuf_size - padding;