can: j1939: transport: add j1939_session_skb_find_by_offset() function

can: j1939: transport: add j1939_session_skb_find_by_offset() function

Sometimes it makes no sense to search the skb by pkt.dpo, since we need
next the skb within the transaction block. This may happen if we have an
ETP session with CTS set to less than 255 packets.

After this patch, we will be able to work with ETP sessions where the
block size (ETP.CM_CTS byte 2) is less than 255 packets.

Reported-by: Henrique Figueira <henrislip@gmail.com>
Reported-by: https://github.com/linux-can/can-utils/issues/228
Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20200807105200.26441-5-o.rempel@pengutronix.de
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>