can: af_can: can_pernet_init(): add missing error handling for kzalloc returning NULL

This patch adds the missing check and error handling for out-of-memory
situations, when kzalloc cannot allocate memory.

Fixes: cb5635a3

 ("can: complete initial namespace support")
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>