net: j1939: j1939_session_fresh_new(): fix missing initialization of skbcnt

This patch add the initialization of skbcnt, similar to:

    e009f95b

 can: j1935: j1939_tp_tx_dat_new(): fix missing initialization of skbcnt

Let's play save and initialize this skbcnt as well.

Suggested-by: Jakub Kicinski <kuba@kernel.org>
Fixes: 9d71dd0c

 ("can: add support of SAE J1939 protocol")
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>