media: dvb_ringbuffer : Fix a bug in dvb_ringbuffer.c

The function dvb_ringbuffer_pkt_next in
/linux-next/drviers/media/dvb-core/dvb_ringbuffer.c,
which searches the idx of the next valid packet in the ring
buffer of the ca->slot_info[slot].rx_buffer at
/linux-next/drivers/media/dvb-core/dvb_ca_en50221.c,
has the following problem.
In calculating the amounts of the consumed address of the ring
buffer, if the read address(rbuf->pread) of the ring buffer is
smaller than the idx, the amounts of the searched address
should be (idx - rbuf->pread),
whereas if the read address(rbuf->pread) of the ring buffer is
larger than the idx, the amounts of the consumed address should
be (idx - rbuf->pread + rbug->size). But there exists an
incorrect logic that the rbug-size was not properly added on
(idx - rbug->pread) in the later case. With this commit, we
fixed this bug.

Link: https://lore.kernel.org/linux-media/[e-mail]
Signed-off-by: Yongsu Yoo <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/dvb-core/dvb_ringbuffer.c

···
 		idx = (idx + curpktlen + DVB_RINGBUFFER_PKTHDRSIZE) % rbuf->size;
 	}
 
-	consumed = (idx - rbuf->pread) % rbuf->size;
+	consumed = (idx - rbuf->pread);
+	if (consumed < 0)
+		consumed += rbuf->size;
 
 	while((dvb_ringbuffer_avail(rbuf) - consumed) > DVB_RINGBUFFER_PKTHDRSIZE) {