can: rx-offload: can_rx_offload_queue_tail(): fix error handling, avoid skb mem leak

If the rx-offload skb_queue is full can_rx_offload_queue_tail() will not
queue the skb and return with an error.

This patch frees the skb in case of a full queue, which brings
can_rx_offload_queue_tail() in line with the
can_rx_offload_queue_sorted() function, which has been adjusted in the
previous patch.

The return value is adjusted to -ENOBUFS to better reflect the actual
problem.

The device stats handling is left to the caller.

Fixes: d254586c ("can: rx-offload: Add support for HW fifo based irq offloading")
Reported-by: Kurt Van Dijck <dev.kurt@vandijck-laurijssen.be>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>